I'm not a good explainer or teacher so I got this from a website lol
Select the eraser tool.
In the primary color box, select the color that you don't like that you want to erase. In the secondary color box, select the color you want to replace said exiled color with.
Then hold right click and sweep with the eraser tool over the ugly color.
If your drawing is large, scale the eraser brush as needed.
(dis is for MS btw)