You will most likey have a MC 1.2.5. jar in you bin folder. Just rename "Minecraft-1.2.5.jar" into "minecraft.jar" If you want you can backup the 1.3.1 jar or just replace it completly
"Force update" means to update the game. Unless you have an old 1.2.5 .jar yourself, you cannot downgrade and you must stay at 1.3.1
There's always MCNostalgia if you're really wanting to downgrade. It just patches your current .jar with the old files and removes any newly made ones.
There's always MCNostalgia if you're really wanting to downgrade. It just patches your current .jar with the old files and removes any newly made ones.