You cannot change the public IP assigned because your ISP controls it but there is something you can do/try.

Providers typically use DHCP to hand out IP’s to customer devices. This is why consumer IP’s constantly change. Chances are however that they simply have very long lease times since most providers charge extra for a static IP.

If your provider has the MAC address on your router/modem set to a specific IP, you won’t be able to change it even by trying to expire the lease. To confirm this, as a test, next time you leave that location for days to a week, disconnect your providers router/modem to see if the IP changes when you get back and re-connect. If that works, it means the provider has long lease times for the IP’s they hand out.

If the IP really never changes, then you might be assigned that IP until you give up the service.
Bonus for you. Your next question should be “What can I do with a static IP?”.