You will need the "Distribute XP" or "Manage the distribution of XP" right. If you don't have either of these, someone else with the "Manage the distribution of XP" right can do this. Whoever is altering your XP donation will need to click on the "..." square, it's next to your name in the guild tab. You should see your rights, aswell as your rank and given XP to the guild. Once there, click on the "..." square next to your donation percentage. This percentage is how much of your fight XP goes into the guild. For information about guilds, and how the donations work, click on this link.