You can find the DHCP Diagnostics at Diagnostics → DHCP.
Here you can see all information about your DHCP server. It’s divided into the IP protocol v4 and v6. Both pages have a Ranges and Leases table.
At Ranges you can see all Interfaces where the DHCP server is enabled and their configured Pools. At Leases you can see each lease with its IP Address, Hardware Address and Hostname. The Valid Lifetime and a timestamp at Expire show until when the lease is valid. Last but not least there is the State of the lease.
In the Actions column you have the ability to edit, create or delete static leases, as well as removing dynamic leases from the server.
To change the DHCP setup please go to Services → DHCP & RA and refer to the DHCP documentation at DHCP & RA.