You move the rear wheel forward or back as needed.
There is usually some type of adjuster mechanism and/or scale to make sure to evenly adjust both sides of the wheel. It is important that the wheel stays exactly straight and is adjusted correctly. The axle has to be loosened to move the rear wheel of course, then torqued to spec afterward. Check the manual for the amount of slack that should be in the chain. Don't over-tighten the chain.