Can I raise prices by 10% for just one brand or one category?

Yes. Open Plugins → Bulk Price Update:

1. Set Apply To to By Brand (or By Category / By Tag) and pick it from the list.
2. Leave Match on Include — only these.
3. Change Type: Adjust (increase / decrease). Direction: Increase, Amount Type: Percentage (%), Value: 10.
4. Choose Apply Change To: Price & Sale Price, Price only or Sale Price only.
5. Click Preview to see how many products and variations change, with a before and after sample.
6. Click Apply Price Update and confirm.

Variation prices of the matched products are updated too.