Turn on the Mac, and press and hold command R immediately after you see the Apple logo. The Mac should start from the macOS recovery partition.
You will be asked to select a user you know the password for, select the user, then click Next, then enter the password for that user (must be an administrator).
When the macOS utilities window appears, choose Utilities, then Startup Security Utility from the menu bar.
When asked to authenticate, click Enter macOS Password, then choose an administrator account and enter its password.
Change the security level you want (don't reduce security unless you really need to).
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