This is the most common and easy way which everyone should try as the first option.
Visit your Magento admin backend in a browser, and click the link Forgot your password.
Insert your email address, complete the CAPTCHA, and then press on the Retrieve Password button.
An email will be sent to your email address, containing a password reset link.
Click the Reset Password link, or paste the link in your browser.
You then will be asked to insert a new password. Insert the password twice and click the Reset Password button to save your changes.
Now you are able to log back in with your new admin password.