How to rebuild Transmission

Just to add to @oldturkey03's comment, I've rebuilt an automatic transmission before and you need to be aware that there are going to be a lot of specialized tools needed to do the job, if that's what you have. You'll also need to locate a repair manual for that specific transmission, which won't come cheap. Unless you're invested in doing this as a learning experience, it'll be a lot more cost-effective to either get a used replacement at a wrecking yard, or purchase a rebuilt unit.

If it's a manual transmission, that's a whole different ballgame, outside of my area of expertise, sorry.