So are you wishing to downgrade the firmware to an earlier version?
If so do you know what version number you are wishing to get?
Let us know
If you follow this link you can download the older firmware files for your phone http://limitededitioniphone.com/where-can-i-get-an-old-iphone-firmware/
Just download the version you want and save it somewhere you can find it, Next open iTunes and once open connect your phone than press and hold the shift key down and click Restore at which point yoru mac should ask you which firmware you wish to restore, Just browse to the one you downloaded and let it restore that version.
Let me know if you need more info.
If yo uhave a PC all you do is download the file you want to your desktop then open iTunes and connect your phone.
Once the phone is recognised and shows in iTunes just hold down the SHIFT key and click Restore then choose the file from yoru desktop and follow the prompts to load the older firmware.