How do I redownload minecraft.jar by itself if already paid?

Hi everyone. Not that anyone should care about this, but as I asked in my other question I tried to install PK's boombox into Tekkit even though that's probably not possible and have been losing it over trying to do stuff with it in the regular minecraft too, and I am just laughing at how I do this to myself, but it's not funny when you're obsessed so don't mention it and let's get on to my question.

I am just saying what I said above because I must be really jaded to minecraft tutorials by now, and everyone says either 1) force-quit-or-whatever minecraft 2) reinstall minecraft or 3) delete META.INF--maybe not on the tutorials but in help--and I basically just want to re put in the minecraft.jar file in minecraft under application support BUT I CAN'T and I've also tried to reinstall the other kind of minecraft.jar, whatever it's named, from minecraft.net and that didn't work either, so if you don't think any of these work and don't have a straight up answer, don't answer this question.

thanks, sorry about the run-on sentences, i DO want this question answered so go back and read again.

snakeramfatale89