You will find in this article some tips for choosing strong passwords and thus protect your accounts: How to choose a strong password?
To change the password, go to cPanel > PREFERENCES > Password & Security
Enter the current password of the cPanel account, the new password, confirm it then click on the button Change your password now!
You have, on the page that appears, a password generator and tips and suggestions that will help you choose a good password.
If you have lost the current password, you can reset it from the login page using your email address.