To increase the quota limit of a mailbox:
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 quota) > Increase the value in the "Allocated Storage Space" field > Click the "update email settings" button.
Please be aware that the parent disk space limit of a hosting account takes priority of the quota limit of a mailbox.
