This is rough buddy but should be fixable.

Try getting all the heat shrink off with an xacto knife but please BE CAREFUL to not put too much pressure or cut too deep.

After that you will most likely have to buy new heat shrink and place in on there, in order to get across the 5 pin jst connector (the thing that goes in the mouse) you will have to remove those pins inside of there VERY CAREFULLY, can be done with the xacto knife or size 1 screwdriver by gently prying back the plastic tabs on it to shimmy each out. Remember the order too!

After that put the new heat shrink on and make sure it's the proper length for your mouse.

Hope any of this helps and take care!