yeah, i overtightened one bolt and snapped up inside seat, but the cover isn’t removable. what can i do, can i leave it?
You can remove the cover by pulling all the staples out aslong as you have some way of restapling it back afterwards. You could always buy a fusion seat cover to replace the stapled one that you have removed, they’re a tight fit btu they wil go over the standard seat foam. If you ride hard on only 3 bolts you will eventually destroy the seat much quicker than if there were four, and the seat will flex more aswell. Basically if you don’t want to risk home surgery on your seat just ride it until it breaks and then get a new one, not muhc else you can do.
I have one of those staple guns, the silver kind you squeeze to put a staple out. I replaced the cover on my KH seat recently because my seat cover broke. Took five minutes to staple it back on (using new staples). I wouldn’t do it more than once or twice because you’d weaken the plastic where you have to staple it back in, but it will work for a one-time repair. Just make sure you get it right 
If you want you could take off the cover, then make a new cover. If you use the search function there are quite a few cover threads. That is probably the easiest and the fastest way of doing it. It is definatly the cheapest.
assuming that its a KH style seat (and i’m not only referring to the fusion), the cover IS removable (as other people in this thread have pointed out)… pull out the staples and put new ones in when your done, i have done this and its very quick.
the new bolt was cheap as hell from the local hardware and stronger than the original. while your at it you can make other seat mods 
of course, there is some saddles which the cover is clearly NOT removable… but then its not worth keeping anyways
One of my saddle bolts snapped off a few months ago and apart from a bit more flex it is fine.