Your iPhone keeps joining a guest network whenever you pass the building. You want to change that one habit, not set up every saved network again. Apple offers two useful choices for a known network: forget it, or keep the password and stop joining automatically.
Keep occasional access or remove the saved network?
Turn off Auto-Join if you still use the network sometimes. After disconnecting, you can join it manually when needed. Forget the network if you want to remove its saved password too; make sure you can obtain the password again before doing so.
On iPhone or iPad, open Settings → Wi-Fi → Edit, then the information button beside the saved network. Choose the relevant option. On a Mac, look under System Settings → Wi-Fi → Advanced, then open the menu beside the network. Labels and placement can differ with the system version.
Check the effect on your other devices
Apple says that forgetting a network also removes it from other devices using iCloud Keychain with the same Apple Account. Do not assume this change stays on the phone in your hand. Some managed networks and supervised devices do not offer these controls; ask the administrator if the option is missing.
For a simple check, disconnect and return within range, then inspect the network actually selected. The old network may still appear among available networks: forgetting it does not hide its signal. You do not need to change the router's shared password just to stop your device from joining automatically.