Had a small leak, which has caused some of the paint around my kitchen windows to bubble and sag. The leak has been fixed but I'm wondering if there's a way to "tighten up" the paint or if I need to rip it off and repaint
Remove all loose paint and repaint. The leak most likely damaged the surface (presumably drywall, but it can happen with wood, etc), so the paint will not "stick" properly until steps are taken to fix it.
For better results, I'd also suggest using a quality primer- something like Killz or ZIN stain-blocking primer, especially if the leak caused discoloration on the surface of the wall. Those discolorations have a way of showing up on the surface after painting if you don't take the steps to stop that.
You'll probably have to prime and paint the whole wall (just the impacted wall, not the whole room) to get a nice, uniform surface, but you may be able to get away with just touching up the impacted area. Hard to say without pictures, and would be dependent on the size of the damaged area, how old the original paint was, the lighting conditions, and so on.