2 cm isn't terrible, but the first question you need to ask is...why is the tank not level? Is the floor uneven where you have the tank? Or is it a furniture (stand) issue? Sometimes tanks become unlevel because they are placed upon a piece of furniture not designed to hold the weight of an aquarium, and over time the wood starts to warp under the weight. The problem with this scenario is that the problem tends to get worse & worse over time, and that 2 cm difference today might be 2.2 cm in 6 months, 2.5 cm in a year, 3 cm in two years, etc. And at some point, either the furniture can't take it anymore and the furniture itself collapses, or else the difference in pressure on the glass due to the extra water weight causes the glass to shatter and/or one of the seams connecting 2 pieces of glass to give way, and you have what NASA calls a "catastrophic failure."
The glass & the seams on an aquarium are designed to safely hold the pressure of the water when the tank is level. The more unlevel a tank is, the more disproportionate the force will be and you will start putting more force on some parts of the tank than those sections were designed to hold. Usually there is some safety tolerance built in, but if the difference in level (and thus pressure) becomes great enough, the tank WILL fail. I had a 29g tank that because unlevel due to warping of the wood furniture it was on, but the difference never got more than 1 cm or so, so I ignored it.
Honestly the best thing you can do if it is at all possible is to identify the cause of why the tank is unlevel, drain out most of the water (to reduce the weight), and then re-set the tank so that it is level. Even with most of the water drained out it will probably take 2-3 people to do it. If the floor's the issue, move the tank & stand to somewhere else in the room. If it's on an a piece of furniture then you may need to bite the bullet and just buy an aquarium stand designed to hold the weight.