That is very simple, just open Krita’s Brush Editor, select a brush you want to rename and then click on the pen icon on top of the Brush Editor.
Michelist
Add/Edit: I have moved your topic into the correct category (or better fitting category, because Bundles and resources help would not be wrong either).
In the future, please try to find the right categories for your new topics yourself, this will help to avoid unnecessary maintenance work.
Each main category has a description for what kind of topics it is meant, the one you had chosen was General Forums and its dedication is described as “General off-topic discussions and casual conversations”,
but your question was a request for support which would fit into Bundles and resources help as well as the category Basics & Interface I’ve chosen for your question, both are from the main category Support and Advice which is meant for →
Basics & Interface dedication is described as “Please ask questions related to basics of Krita and its interface in this category.” and I have chosen it because you seem not to be familiar with Krita’s interface, and the dedication of Bundles and resources help is described here About the Bundles and resources help category.
You can find these “About the XYZ-Category” topics in every sub-category of the forum, and they are meant to help you find the correct categories for your questions.