There are a few ways to keep your cat calm:
First look into products like catnip or Feliway spray. Some cat's don't respond to catnip (mine for example), but alternative products like Feliway have a huge effect. When I use Feliway, she becomes so relaxed that she sleeps. These products are good for short term but fast effects. You could also ask if the vet has a product (like food supplement) that you could give when you need to remove the bandage.
If you can't get your hands on commercial products like Feliway or catnip, you can try adding some chamomile tea into their food. Please remember to check with your vet before giving the cat any supplements, especially if they are on medication (as some supplements can disrupt effectiveness of medication).
Secondly you can train your cat to become more calm when you touch her belly. This doesn't always work, and can take weeks (so not ideal for your situation). It's still a good idea to do for future situations. Let the cat get relaxed near you, and gently stroke her side. Over time she will become comfortable with this, and you can begin moving your stokes closer and closer to her belly. Eventually she/he will learn that touching the belly by you isn't a threat. Pay attention to their body language - getting stiff, flicking tail, folded back ears, etc. Speak calmly and constantly, and give lots of little treats afterwards (many cats will sit up when eating treats, so best to do after so you have access to the tummy).
Finally you can do techniques like "Burrito" the cat in a blanket. I haven't tried it personally, but I know some people swear by it. There are a lot of Youtube videos on how to do this. Try it beforehand to see if its effective on your cat.