how to change login pw in imac

Hello!

Please see:

Change the login password on Mac - Apple Support.

If you forget your password, please see: If you forgot your Mac login password - Apple Support.

If you have tried those steps and you cannot reset your password, see: If you can't reset your Mac login password - Apple Support.

Jack

1. On your Mac, choose Apple menu > System Preferences, then click Users & Groups . ... 
2. Click Change Password.
3. Enter your current password in the Old Password field.
4. Enter your new password in the New Password field, then enter it again in the Verify field.