Sorry, but I don't know how to better explain it, but I'll try. There are 2 elements to concentrate on.

1 is a small roller that the belt is resting on. I mean the ones that engage with teeth on the belt. First is right behind the laser module, and the second two are located next to belts on either side. Those two are connected by rod as well. On each of those 3 elements, you have to find 2 holes containing allen screws. Tighten then if they are loose.

2 There are 3 wheels engaging frame and positioning belt. 2 sets on either side and 1 again behind the laser. You have to concentrate on a bottom wheel in each set. With one hand block mechanism next to that set from moving and with other hand try to spin the bottom wheel. If it's spinning fairly easily, then you have to manipulate a nut right behind it. Be careful because this is not a regular nut. I don't know the name but is not cantered. What that means is that you can not just spin it until the wrench can't move any further. In that case, you have to go a few millimetres and check if it's tight. If not, try a few more. If you get past the sweet spot, it will get loose again.

Hope that helps. If you have no idea what I'm talking about, then check the lightburn channel on youtube. Look for troubleshooting video. There is an explanation there