how to access imac with OS High Sierra f you forgot your password

after 10 years of using my iMac i must have forgotten or changed the password when i got the new Macbook Pro Laptop. there are a couple of important things left on my old iMac and i can't access it anymore. I tried everything but the password recovery after three failed attempts does not work here.

Any idea how to get into my iMac or change the Password?

thanks for your help