Editing a retrospective is very easy, you can edit any setting you like from the Settings page. This will allow you to change the next retrospective dates and any other settings like the retro format or the health checks.
In order to do edit your retrospective just click in the 'edit' button on the top right and once done make sure you click 'save'.
In the case your retrospective form for that particular retro has gone out already, then you will only be able to edit the meeting date.
Bear in mind that only Admin users will be able to edit the team settings.