Are you using Pi-hole v5, or v6?
If v5, you can modify the port lighttpd is using via the ‘server.port’ configuration setting in /etc/lighttpd/lighttpd.conf.
I found this information via a search, and have not verified it myself. If you change the port, you will need to restart the lighttpd service for the change to take effect.
I know it can also be changed on v6, but not through lighttpd, as v6 does not use lighttpd.
Also note that there is an earlier post to this forum from back in 2021 indicating that lighttpd.conf could be overwritten by Pi-hole, and should not be changed (https://discourse.pi-hole.net/t/how-can-i-change-the-default-port-of-pihole-changing-lighttpd-conf-not-working/47560) . Not sure if this still applies, as I do not see the warning in the current lighttpd.conf file. I still use v5.