How do you change a freeze plug leaking under water pump ?

Is it dripping from the waterpump? Gradually getting worse?
It will appear to come from the back of the water pump. This means that you'll have to change out the pump itself.
Haven't actually done a sonoma..but basically the same on most vehicles. Pick up new pump from auto parts store. Some gasket sealer too is good idea. Also some anti freeze.

Drain antifreeze into bowl to reuse. Run it through a sock or other filter befor putting it back in.
Loosen and remove any belts. Remove the small bolts holding the pump to the block.
Possibly you may need to move stuff like the alternator to reach it??
Look at the new pump to see how many bolt holes, so you don't miss any.
Pull pump away. May need to tap the pump with hammer to bust it loose.
CLEAN the mounting surface. CLEAN the mounting surface. CLEAN the mounting surface.
Avoid gouging the surface. Apply film of gasket sealer to CLEAN surface, AND the gasket that came with the new pump. Block, Sealer, Gasket, Sealer, Pump. Position pump and bolt in place evenly. Snug up each side at same time step by step. Tighten firmly,but don't crank down too hard, you'll strip threads.
Restore any moved items, replace belts and tighten all up. Fill radiator. Start engine. Top up radiator. Top up again with gentle reving of engine. Close radiator. Fill top up bottle.
close hood if all seems ok.
Good luck