There may be times where without really thinking through the consequences we might click "Save Username" on a public computer, this isn't such a bad thing, but if you also allow the browser or third-party password manager to save the password then you have an issue that you need to deal with.
The answer to how to fix this is dependent on the browser or password manager in use, but essentially it is simply going into the browser settings, going to the password section and deleting the entry. Here are links to clearing a saved password from the primary browsers on the market, if you are using something different we will assume that you are technically savvy enough to figure this out.
Chrome: https://support.google.com/chrome/answer/95606?hl=en&co=GENIE.Platform%3DDesktop
Safari: https://support.apple.com/en-ca/HT211145
Should any of these links fail (they were working at time of publishing) simply do a Google search for: "[Browser Name Here] delete saved passwords"