take dizzy cap off before you start and keep an eye on the rotor arm, if it moves put it back. then just loosen the tensioner and pull the belt off the cam sprocket, move cam round with a spanner/socket on the cam sprocket bolt and refit the belt + tension the tensioner. spin engine 360 degrees and recheck timing
but you'll probably need to do the dizzy timing to as per my digifant tuning guide if the cam timing is out by that much.
oh and if you don't have a tensioner tool, what i use is an allen key. fit your socket to the tensioner nut, then fit allen key to one of the tensioner holes. brace allen key to the socket, and push down to set your required tension, nip up the nut. Once you turn engine over 360 degrees recheck your tension, you want to be able to twist the belt around 90degrees at the front length below the cam sprocket
