One way to do it is press the button inside the machine situated on the small circuit board where the coins enter but that doesn't give you unlimited credit.
For unlimited credit you need to programme it:
When you open the front, up by the small strip light at the top is the maintenance/service switch. Pull it out and the display will change to P ##.
Press C on the selection pad.
Then press 090 H 0 H to cancel any active time
windows that might already be set.
Then press 094 H 200 H. When you have finished push the maintenance button back in and the credits should show as 99 and should stay at 99 even when a selection is made.
If you have a
remote control with a credit button you should be able to switch between free credit and paid for credits by pressing this button.