There is an alert banner at the top of Aria Operations 8.18 that has a "MANAGE YOUR LICENSE" button, which navigates to Subscriptions>License Management.
The expired certificates are unable to be removed because of an alert that shows up when attempting to use the "REMOVE LICENSE" button:
Your selection contains license(s) that cannot be removed. To remove those license(s) from VMware Aria Operations, you must first remove them from vCenter.
VMware Aria Operations 8.18.0
This is caused by any of the configured vCenter's inventory having an expired, or expiring license.
For (each of) the vCenter(s) that is(are) configured in Administration>Integrations:
Once the expired or expiring licenses have been remediated in the vCenter inventory, they should be removable in the Aria Operations interface once the change is synched after a few minutes: