I don't know why everyone is telling you to go straight to a VAR, you should try and learn the equipment and then when you're not comfortable with whatever the task at hand is, then you should reach out for external assistance, running directly to a reseller everytime you have a minor issue isn't going to help you to learn.
Anyways, first thing you'll need is putty this is generally what's used to access the devices if you're desktops on windows, find the IPs and credentials of the devices in the documentation and use that to access the device, in putty it defaults to SSH, if this wasn't set up properly the change the checkbox to telnet and try again. This program can also be used to access the physical console ports on the devices by using a serial cable. IOS is the operating system that's actually running on the devices, you need it on the networking equipment to access it but not your computer :)
Assuming the devices have an active support contract you should be able to link your Cisco ID to the contract numbers in order to obtain authorization for the downloads, from there you can usually find the reccomended realeses for each device, just make sure they have enough hardware to run them (can be verified using the 'show ver' command) as over time resource requirements increase and if you have an older hardware revision and haven't/can't upgrade you might need to go back down to an older string of code. Generally unless you really need a feature stay away from T train builds as they are more inclined to contain bugs and aren't supported for as long as the normal builds are, eventually all of those features will get merged into the normal code anyways. That being said read the realease notes everytime you upgrade, espcially this time, there may be multiple code hops you need to do in order to successfully upgrade or you device may have to reprogram some of the firmware in the hardware, even if everything comes back OK those 30 minutes of praying the device will come back won't be enjoyable if you're not expecting it to happen. All that being said, I wouldn't necessarily have someone onsite for your first upgrade but I would HIGHLY suggest starting a relationship with a VAR so you have someone to call that you've talked with before in the event an issue comes up. Cisco TAC is also extremely helpful so if you have a support contract you can reach out to them as well.
Contract association:
http://www.cisco.com/web/Downloads/SDS/unentitled_instructions.html
Software Downloads:
https://software.cisco.com/download/navigator.html
There's also a few GUI management tools if you are more comfortable with that, it doesn't give access to all of the features of the devices and some additional configuration maybe required on the devices before it's usable; CNA is available with without having to purchase it:
http://www.cisco.com/c/en/us/products/cloud-systems-management/network-assistant/index.html
http://www.cisco.com/c/en/us/td/docs/net_mgmt/cisco_network_assistant/version6/guide/gsgcna.html
For the aironet APs since you are fairly new they should have a WebUI you can access if they're autonomous (no controller managing them), otherwise you'll need to access the controller to make changes to the APs.
Here are the some quick resources to help you get up to speed while you learn more, they may not be an exact match but as long as it's in the same series it should be pretty accurate:
Switches:
http://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst3750x_3560x/hardware/quick/guide/3750xgsg.html
Router:
http://www.cisco.com/c/en/us/td/docs/routers/access/1800/1841/hardware/quick/guide/1800qsg.html
Wireless:
http://www.cisco.com/c/en/us/td/docs/wireless/access_point/1140/autonomous/getting_started/guide/ap1140aut_getstart.html
So now that the basics are covered the ISP change I'm assuming you have a static address otherwise everything should have worked properly (it's hard to know if that's actually the case without a config though), if you do have a static you'll need to change your outside interface IP address, then you'll need to go and change your NATs and Access lists to reflect the new network, after that you'll need to change what ever routes you have to point to the new ISPs router. If you have VPNs this get more complicated and you will need to work with the other end to get this changed on their side as well as yours. Simply put, anything that references the old ISP will need to be modified to use the new ISPs network.
As I said at the begining though please reach out to external help if you are not comfortable with any of the changes or troubleshooting a failed OS upgrade, this post is not to be used as a reference guide or a garuntee of no complications but merely to try and help you get pointed in the right direction. Please verify everything for yourself in the documentation before doing anything in production.
HTH
