Hello Ashley,
you can follow the step by step guide at this link
http://www.cisco.com/en/US/docs/routers/access/1900/software/configuration/guide/upgrade_ps10538_TSD_Products_Configuration_Guide_Chapter.html#wp1054561
You need a TFTP server running on your PC, IP connectivity has to be present between the PC and the router, the IOS image has to be located in the working directory of the TFTP server (or the TFTP working dir has to be changed to the path where the IOS image is present)
The flash memory of the router needs to have enough space to save the new file.
Hope to help
Giuseppe
