does it have the standard metal belt tensioner ?
Looks like the spring that works a clothes peg.
If so just take that off to remove belt tension, adjust till you think it's right and put the tensioner back on.

At least that's how I would do it.

And the reason for the questions is that a lot of people will leap at a printer with a tool kit, when there's nothing wrong with it and the issue is often something simple entirely unrelated to the hardware setup :-)

We like to cover all bases before telling someone to start dismantling a working printer.

Given that most questions are asked by people fairly new to their printers, it's an approach that we have found works.
Sorry if it's annoyed you, but most people just don't give enough information for someone, who has not seen the machine in question, to base a proper answer on :-)