Depending on the type, some transmissions can have multiple gear ratios. Multiple geared transmissions are typically automatic, meaning the transmission will shift on it's own, but, while rare in the United States, a manual shift transmission can be found on some mopeds.
Included in the transmission on some mopeds is a variator setup allowing for continuously variable shifting. Use type A, or F, Automatic transmission fluid (ATF). You can pick up a bottle at any autoparts store. On a Tomos A35, to change gear box oil you drain the plug or bolt at the bottom of the transmission. The bolt on top of your transmission is for filling. There is a phillips head screw on the side unlike the allen heads that actually hold on the gearbox cover. Once you have drained the old oil, and bolted back on the bottom plug, fill new transmission fluid until it leaks out of the side where you have taken the phillips head off. Many recommend being very careful NOT to strip out the phillips head on the side, as it is not very strong metal. Use an appropriate sized phillips screwdriver.