Login in to your cpanel (usually https://domain.com/cpanel - you'd need to replace domain.com with your actual domain), then click on:
Email accounts > Manage (on the same line as the mailbox for which you'd like to change the password)
Enter what you'd like to be the new password in the "new password" field, then click "update email settings".
Please be aware all mail clients connecting to the mailbox for which you've changed the password will need to be updated with the new password to be able to send and receive emails via the mailbox for which you've changed the password.
