First step is to log in to your hosting control panel (cPanel).
Here you'll be able to adjust many aspects around the domain hosting, databases, DNS and also email settings.
Within the hosting control panel you'll see an Email section.
Here you can create new email boxes, add forwards/redirections and even set up auto responders independent of your email program/s.
Under Email >> Email Accounts is where you see all the active email mailboxes.
You can 'Check Email' (webmail through the browser) or 'Manage' the details for each email account.
Email setup instructions is under the 'Connect Devices' section as well.
To change the password for an email box account,
Go cPanel Email >> Email Accounts >> 'Manage' (for the emailbox that you need).
In this detailed screen you will be able to set a new password, check your webmail, adjust your mailbox quota and even get specific details for connecting to your devices.
** Remember the 'Update Email Settings' button at the bottom when you are done.
You can use the 'Check Email' or 'Free up Email Storage' to clear up some of the email off the mail server when your mailboxes are getting full.