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

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'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

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