Can I change the functions of buttons and knobs to suit my preference?

This isn't possible when using Serato DJ Lite.
You can do it when using Serato DJ Pro, but you'll need to buy a Serato DJ Pro license or take out a subscription for the software.

Serato DJ Pro has a Remapping feature. You can use it to change the buttons or knobs that features are assigned to. For details, see the Serato DJ Pro software manual on the Serato website [serato.com].