If you still have the old IOS in the router's flash the downgrade is quite simple. You can force a router to use a particular IOS image in the flash and load it in the RAM (boot system commands). However, if your flash did not have enough space to hold two IOS image files and the older IOS is no longer in the router flash then you will have to follow the same procedure that you did to upgrade the router, however, this time you will be downloading the older IOS image in the flash rather than a new one.
I hope it is clear. If not then do let us know and we will try to explain it in more detail.
