Releasing DHCP Reservations

Not a seasoned Network Guru, so apologies ahead of time if this question is obvious to most…

How do I release a DHCP reservation for a client that is no longer in use. I have the leases set up to the default, for one day, but if those devices are offline for more than a day, do the IP addresses go back into the pool? For example; I just upgraded the family’s phones. The old ones had all Static IP addresses. When I set up the new phones, I used new IP addresses. How do I tell the PepLink Balance 20x to release the old IP addresses to be reassigned, as I don’t see an option for this, or does that just happen on it own? I can see in the Network>Lan option that while the device is online, you can hit the “X” to delete a reservation, but how will this happen if the device will no longer be online, showing up in this window to cancel?

Thank you for the help,
Rob, MI