If you wish to do fixes to your Hyosung RT 125D then it is usually a great idea to have a Workshop Manual to keep just for a reference in case that you need to know techniques to strip off a specific component including Brake Discs and fit it again safely.
Hyosung RT 125D Workshop Manuals offers you distinct diagnostic guides for the vehicle so if something is faulty it will help you discover what the situation is and ways to repair it. After that you can decide if this can be achieved on your own or that the job is just too big you could make a reservation for your RT 125D in with a suitable friendly mechanic.
Hyosung RT 125D manual