The thing to understand about keychain and passwords under macOS is they are stored in several places:
- Cached within the browser
- In the "Local Items" Keychain
- In the "Login" Keychain
- Optionally in iCloud
Assuming you have iCloud turned off for passwords (lets not make this any more complex and error prone than it already is) then you might expect you could just edit/delete passwords in Safari and /or Keychain access and the machine would just figure things out and sync everything. i.e. how they work in Firefox.
In my experience this often does not happen, you delete a password in Keychain and Safari will 'helpfully' save its cached password back to the keychain a few seconds later.
You delete a password in Safari and it will 'helpfully' re-import it from Keychain a few seconds later, and so you are getting nowhere.
The trick is to close Safari completely (don't just close the window, press Command-Q to completely exit.)
Then edit or delete the password using the utility Keychain Access and wait a few seconds before opening Safari again.
There is one extra wrinkle to this, if you have ever updated macOS to a major new version, the keychain permissions can get messed up which will prevent you from deleting any passwords that were migrated from the old install.
To fix this in Keychain Access, double click on the entry and tick 'Show password', Edit it to be something different and save. Now you should be able to delete it.
Many website passwords will have 2 entries, one under the "Local Items" Keychain and one under the "Login" Keychain (I have no idea why), you need to delete both.