Pop up to enter password keeps coming up with old Apple Id and doesn't allow me to change it

Hello Anne McC,

This page can help out with providing steps for signing out of the iCloud account on Windows: Sign out of or uninstall iCloud for Windows - Apple Support

Note: Before signing out of or uninstalling iCloud for Windows, you may want to make a copy of your iCloud data and save it on your PC. The information on your PC won’t automatically stay up to date in iCloud, and you won’t see updates made on your other devices.

Sign out of iCloud for Windows

When you sign out of iCloud for Windows, the information you store in iCloud is removed from your PC. You can still access it on your other devices and on iCloud.com. See Where else can you access the data you store in iCloud?

1. On your Windows computer, open iCloud for Windows.

2. Click “Sign out,” then follow the onscreen instructions.

3. If you decide you want to access information stored in iCloud on your PC, you can sign in again.

Cheers.