Here is a link to download the firmware:
https://software.cisco.com/download/release.html?mdfid=282414013&softwareid=282465789&release=1.3.3.6
No need for incremental updates. This should preserve your settings but you may want to write them down just in case.
There are more recent versions however it looks like your router is version 1, a version 2 firmware won't work on it.
To upgrade you download the file then login to the router usually 192.168.0.1 or 192.168.1.1 in your web browser. Login default is admin/admin
There will be several tabs 1 of which will either be administrator or system management the option you will be looking for is Firmware upgrade. Click the button that says browse and select the file you downloaded then click upgrade. Give it some time to work. Don’t unplug it until it has finished. It should only take a minute or 2.
~chris