The water pump is usually at the front of the engine, so you can reach it from underneath. Jack the car up and put stands under it to be able to reach...never work under a car held only by jacks.
(You might be able to find a very deep gutter you can drive over, too, to give yourself working space)
If a seal has half come out, it is probably broken and perished. Get a new one, they are not expensive. Also get a tube of something like GasketGoo, a sticky sealant in a tube that will hold the seal in place and fill any gaps.
Put all pieces onto a tray in the order you take them out, and do the job in one day so you don't forget what went where.
Disconnect the hose to the water pump first and drain out the water.