On the router you are wanting to change the IOS version for copy the config to the flash: storage.
In my case I have the file on a USB pen so first I get the exact filesystem name
show file systems
Then copy the IOS from the pen to the flash storage of the router:
copy usbflash: {MYIOSNEW} flash: {NEWLOCALNAME}
Enter configuration mode:
config t
Clear all boot operations configured inside of startup-config:
no boot system
Now change the boot rom from inside of config terminal:
boot system flash {NAMEOFROM}.bin
Save the changes:
copy run start