Hi -
If you mean re-use a different section of a track [like, say, re-using a line from chorus 2 in place of the the same line in chorus 1], then you would cut up the Region by positioning the cursor and hitting Command-T [Split Regions at Playhead] to cut out the section you want, then drag [to move] or Option-drag [to copy] it over to the place you want to move it to. You can trim the beginning and end of the cut section with the Trim tool [the little brackets at the lower corners of each Region].
If you mean re-recording over a short section of an already-recorded track, replacing just that section with a new recording.. Well, that's called punching, and GB doesn't do that as easily as other DAWs. While you can record over a section on a track, it seems like you can't audition the previous/following audio on the track as with normal punching in/out, so I think the easiest approach in that case may be to record the desired section/sections on a new track, and then cut/edit the desired section into the original track, as above. [You can also record on the existing track to record an additional Take by enabling the [yellow] Cycle bar and then edit that, but you might want to get used to GB a bit more before you try that]..
I've attached a GarageBand course to this thread [More Info, at the top of the thread] -- videos 17, 20, and 24 seem to go over basic recording and editing techniques, and video 31 covers Take recording/editing, if any of that's any help..
