How do I modify a printer's IP address in CUPS?

You can edit /etc/cups/printers.conf

In a terminal:
sudo systemctl stop cups

sudo vi /etc/cups/printers.conf - (or the text editor of your choice, I chose vi.)

locate the printer's URI (in bold below), it will be among all of your printers' profiles, they are arranged within tags like:

<Printer My Poop Laser Printer>
PrinterId 1
UUID urn:uuid:c41ca0c7-52f3-3448-5dfe-11281fc3ac2e
AuthInfoRequired none
Info
Location
MakeModel Generic IPP Everywhere Printer
DeviceURI ipp://192.168.200.2/printers/laser
State Idle
.
.
</Printer>

You can edit the IP there, save it.
And then restart cups...

sudo systemctl restart cups

I've never had much joy from the CUPS web interface.

You can change the printer URL here, at least if your setup is similar to mine.

Preferences/Printers, right click and choose Properties

The IP address is not a modifiable field, because, if you changed it, it would describe a different printer.

Abandon the "Modify Printer" approach, and on http://localhost:631/admin, Find New Printer

I just went through the same exercise when my subnet changed. You can modify the connection. However, you will not be able to select current connection. You have to select a "new" connection, i.e., LPD or AppSocket or, for you, ipp. As your original/current connection is ipp, chose Internet protocol printing (ipp). Hit continue and fill out details the as current connection.