I can't be specific because you didn't tell us what type of DSLR camera you have.

It depends upon what you expect the new focusing screen to do for you and which one you get.

The main purpose is to get one that has horizontal and vertical markings on the viewfinder screen to help you align buildings and other vertical/horizontal elements in the scene you are viewing.

That keeps you from accidentally tilting the camera while using the viewfinder.

Most people who get a replacement focusing screen like it better than the one that comes with the camera.

The actual replacement is a bit tedious.
If you are all thumbs, don't even try it.