You'll be able change your invitation design any time before sending the invitation. You can do this by using the Change Design button located at the top of the Design page.
You'll then be able to select a new design from our selection. After finding the design you like, click the "update design" button to change your invitation design.
All of the event information will be transferred over to your new design.
You can also use the Change Design link located below the invitation your Dashboard as well.