When logged in, changing your admin password in Magento 2 is quite simple.
The following steps are:
1. Click on your account name at the top right corner and click on the 'Account Setting (Admin)'.
2. Then enter your new password in 'New Password' field and confirm this password in the field below ('Password Confirmation'). Always use a long and complex password!
3. Enter your current (old) password in 'Your password' field below 'Current identity verification' section to confirm the changes.
4. Click on the 'Save Account' button and you're done!
Tip: Change the password of your admin account periodically. Use a password manager such as Lastpass or Keepass to easily remember these unique passwords.