Unless you remove the tree, dig out the roots and get rid of it all, nothing you can do - it happens as trees mature.

Welcome, Gedoja!
Sadly, that is something that you can sometimes prevent (but not always) but can't fix once it happens. You could switch from lawn to a ground cover where the roots are a problem, but that may not be useful if you have children and a small lawn.

I've seen the problem successfully tackled by adding soil on top of the roots, and re-seeding/turfing over the top. Otherwise, however, as above, you'll have to live with it, or remove it and live without the tree!
Phil J