Leave guild during the war
From Realera Wiki
Basically you have 2 options to do that:
1. If no new guild war has been accepted, you can leave the guild after 3 days.
2. Order name change in our store, go Game Store (website) -> specials -> Character Name Change and don't forget to check the option to kick from current guild. (cost 300 points)
or alternatively wait until the war is over.