iMac Password
I forgot my iMac password, Not my Apple ID password. How can I reset the password for my iMac?
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Okay, as I understand it, then, your Mac is set to automatically log you in at startup and you have forgotten your Admin user password.
The link below details how you would normally change your user password, but it does require that you remember the original. The automatic login can be canceller at System Preferences > Users & Groups, but you may be asked for your admin password to unlock the panel.
Change the login password on Mac - Apple Support
What Mac model and year do you have?
What macOS version are you running?
Find that info by clicking the (Apple) menu at the top-left of your screen and then select About This Mac.