How do you leave a guild in Castle Clash?
Go to your guild hall and press the "Guild" button. On the bottom right corner of the top section of the page where it explains the Guild's information (credits, name of guild, etc.), there should be a button that says "Leave". If a pop-up box does appear asking if you are sure, click yes and you are no longer part of that Guild unless you rejoin the Guild.
After leaving the guild, there will be a 24 hour cool-down that will prevent you from joining another guild.