Maintenance for the week of January 12:
• [IN PROGRESS] ESO Store and Account System for maintenance – January 13, 9:00AM EST (14:00 UTC) - 12:00PM EST (17:00 UTC)
When you open your inventory, there's an icon for the disguise on your equipped gear window. It's next to the jewelry you have equipped. Just unequip it the same way you would do with any other piece of gear.