Can I replace this breaker that keeps heating up and tripping?

Hello. This breaker has never given me trouble, but now keeps tripping at importune times. I'm GUESSING it just finally met it's match, after a couple months of having a portable air conditioner plugged in during the day. I was an electrician apprentice many many years ago, but that was high voltage, working at a coal mine, so experience doesn't help too much.

It is tripping, and getting quite hot to the touch. Wondering if I can just change out the breaker to fix this. Do I need to change the one next to it, also? (This residential building is from the 1970s, in Denver, CO, if that helps.)

Pictured are the box, the breaker (bottom left, with red wire) and another breaker that was previously replaced from this box.

Thank you!