On this computer, it doesn't have an admin at all, or it has an admin, but you don't know the password?
If it doesn't have admin at all, there's a way to recover admin access.
Hold down Shift key while you choose Windows restart, then go to Troubleshoot > Advanced Options > Startup Settings, click on restart, then choose option 4.
When prompted for password, leave it blank and enter.
When you are in safe mode, hold down windows + r keys, type in netplwiz and press enter.
Then double click on your username, go to Group Membership tab, make it admin.
Restart, log back into your user. You should now have admin access.
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If it has an admin user but you don't know its username or password correctly, you will have to back up files and perform a clean install. No other way around this.