If you're experiencing a gear malfunction it could indicate the need for adjusting. You can quickly try to adjust them yourself by doing the following.

1. Choose gear 4. 

2. Locate the black thingy between the gear cable and gear lever. Pull it out a few millimetres towards the cable and adjust.

3. Look at the gear hub on the right side of the rear wheel. Locate the two yellow lines, these should be aligned when the gears are adjusted properly. To adjust the gears, rotate the black thingy back or forth on the gear lever until the yellow lines are aligned.