You can upgrade OWA-only mailboxes to full-functional Exchange by going to CONTROL PANEL > Services > Mailboxes > OWA-Only Mailboxes > click on a mailbox > Upgrade to Exchange.

upgrade to Exchange

Note: the feature availability depends on your plan. If Upgrade to Exchange button is missing please contact Support for assistance with the upgrade.

Read the Knowledge Base article on OWA-Only Mailboxes: Overview for more information.