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

Yes. The way to do this depends on which DJ software you're using.

■rekordbox for Mac/Windows
rekordbox has a MIDI Learn feature. You can use it to change the buttons or knobs that features are assigned to. For details, see the MIDI Learn Operation Guide [here].

■rekordbox for iOS/Android
It's not possible to change the button or knob assignments for rekordbox for iOS/Android on the DDJ-GRV6.

■Serato DJ Pro
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 [here].