Reset iMac to factory settings without Apple ID Password

I have two usernames for my iMAC. A main account and a secondary one I had years ago. I can login to my main account without any issues, but I forgot the password for the secondary account, and the recovery is an old phone number I had while living abroad, which no longer works. I am now selling my iMAC and need to reset it to factory settings, however it wont allow me unless I login to secondary account. How can I do this without having the password?