If you want to undertake maintenance tasks to your Yamaha TZ 125 then it will always be recommended to have a Workshop Manual to keep just for reference just in case you want to know the best way to strip off a specific component for instance the Gearbox and install it again in the right way.
Yamaha TZ 125 Workshop Manuals are able to offer specified diagnostic wisdom for the vehicle so if something is wrong it helps you find out what the problem is and the way to repair it. After that you can determine if you can do this on your own or in the event the job is simply too big you could book your TZ 125 in with a decent garage.
Yamaha TZ 125 manual