Where do you have the plant? Hard to tell exactly from the photo, but I would guess sunburn first of all....could also be some kind of mechanical damage--from a fall or something banging into it....I'm sure there will be other opinions coming :-)

Looks like sunburn.

I have it in a window sill that is facing south, but there is a tree and some shrubs outside that window so I thought it would have some protection. But, never had this problem when I had it in a different window! I'll move it back to it's other location! Should I leave the affected leaves alone?

I would.