Once this happens you will see a message telling you to upgrade your plan, like below:
At that moment, you have 3 options:
Update your backups in a way that you use only the storage quota available in your plan (update your retention policy, purge backups ...)
Upgrade your plan (a higher plan has more storage included)
Reach out to us via chat or email and ask for an additional quota. An add-on will be created to your subscription within 24hours.