Windows 10 comes with a default Administrator account that can be used in lieu of a regular user account when making administrative changes. The account is disabled by default.
The simplest way to activate the Administrator account is via Command Prompt. Launch Command Prompt from the Start menu (search for "cmd"). Right-click the search result and select "Run as Administrator".
Next, type "net user administrator" into the Command Prompt and press enter. You should see a list of data appear - if you look at the "Account active" line, you should the account is inactive.
To enable it, type "net user administrator /active:yes" and press enter. Run the earlier command again to confirm the change.
With the account activated, you'll now be able to access it from the login screen. You can login as Administrator to address issues with your own account or other administrators on the machine.
It's best practice to disable the default Administrator when not needed to mitigate security risks. To do this, repeat the above procedure, substituting "/active:no" into the second command.