How to reset iMac password?

Follow these instructions: https://support.apple.com/en-us/HT202860

Reset Computer Password  for the Administrator Account Password

Intel Computer

1 - Restart computer and immediately hold Command + r keys

2 - Top Tool Bar >> Utilities >> Terminal

3 - Type resetpassword as one word and no spaces

4 - Press Return key

5 - Follow the prompts and remember the New Password

Apple Silicon M1 Computer 

1 - Choose Apple menu > > Shut Down.

2 - Press and hold the power button on your Mac until you see “Loading startup options.”

3 - Click Options, then click Continue. 

4 - Top Tool Bar >> Utilities >> Terminal

5 - Type resetpassword as one word and no spaces

6 - Press Return key

7 - Follow the prompts and remember the New Password