You can edit them 
The easiest way is to go to Project Settings, then go to Input. There will be a Mobile section, with a Default Touch Interface node. You can click the magnifying glass and it’ll bring you to the file in the engine assets. I recommend duplicating it, and using the new one.
Then from there you can make all the edits you want, like size, location, image, right/left only, etc. Whatever you need