You can update your email quota (or size) and password from the same option from where you can remove any email address.
You need to first open 'Email Accounts' page from the control panel:
To change the password for any email address, directly press on 'Passwords' under Actions in 'Email Accounts' option within the page:
Once you click on it, you can see two fields from where you can update your password for the desired email address. Please ensure that the password you enter is strong enough. Once you're done with entering the password in both the fields simply click on 'Change Password' and you're done.
The process is identical to changing quota for your email address, simply click on Quota instead of the password from the same screen. You'll then see Mailbox Quota if you want to restrict the size of mailbox, simply enter the value you want to be for limit (in MB) in the first field. If you want the mailbox to have no restractions on size, simply click on 'Unlimited'. Save your changes by clicking on 'Quota'.