There is a room on the top floor of the medical facility on Coruscant that allows players to renounce a secondary guild.

Primary guilds cannot be renounced.

Half of the experience invested in the dropped guild will be forfeited and players will lose hitpoints for the equivalent number of levels they are dropping in the guild.

Players will retain their skill proficiencies in the guild, but won't be able to use its skills.

Renouncing a guild is not reversible, so make sure you really want to leave a guild before you renounce it.