We recommend adjusting this at a professional bike shop, preferably specializing in e-bikes. 

To fix it yourself, first unscrew the six Allen bolts next to the big nut that holds the rear wheel. You can move the rear wheel by tightening or loosening the two smaller Allen bolts that keep the distance between the frame and the wheel. The belt drive must be able to move around 1-2 cm when you press it. If the belt drive is tight enough, you have to check if the wheel is straight in the frame. If everything is in place, you can tighten the six bolts again to secure the wheel. If you have questions, please reach out to Customer Care.