To downgrade AC:Unity to version 1.4.0 which has all DLCs and has ultrablending glitch you need to change ACU.exe from the folder of the game to the ACU.exe from the 1.4.0 version of the game.

You can download 1.4.0 version of ACU.exe from the Resources section.

I tested it only in offline mode - so i recommend go offline in Uplay before using it - just to be safe.