what are your drains made of?
Abs, you need a t-wye and a coupling.
Cut wall open, cut out old t wye, measure for new t wye, and install coupling the place where you cut out the old t wye. chain grips or a strap wrench will help as you will have to angle or lift the piping in the wall to fit the new parts in... pretty easy job I would give it a 5 out of 10 for a DIYer and a 1 out of 10 for one of us.

you Could also use a no hub coupler in place of the glued coupler ( make sure it is the proper one for the diameter of pipe and type ) and i would likely say its a 3 out of 10 for a diYer then.. though a glued one is better.