Reset/Refresh DHCP server

Hello, Recently we have been getting a lot of " IP CONFLICTS' in our network. Ofcourse iassuming that we are running out of IP addresses, i changed the lease time to 7 days from 3. But we still get the IP CONFLICTS since the DHCP server is unable to renew. How do I clear the DHCP service so it starts assinging new batch of IP addresses.