If you've met and exceeded the offer conditions of your Firm choice, you can use UCAS's Adjustment process to apply for an alternative course.
In 2021, Adjustment is available from A Level results day until 18 August and, if you qualify, the option to register will appear in your UCAS Track page. If you register for Adjustment but you don't find anything, you'll keep your original Firm choice. You only give up this place if and when you confirm you'd like to go elsewhere and the new university adds themselves to your application.
If you qualify for Adjustment you'll see the option to register in UCAS Track.
For more information on Adjustment, visit: ucas.com/ucas/undergraduate/apply-and-track/results/ucas-adjustment-%E2%80%93-if-you%E2%80%99ve-done-better-expected