Whilst you can't extend an existing series, it is quick and easy to create a new series of sessions on your schedule to pick up where your current series ends, presenting to customers like a continuation of the same series. To do this:
Click Add new session on the Schedule page
Click Copy an existing session
Select a session in the series you want to add to
Change the relevant series/date/time information and create the class/series
While technically a different series this is the quickest way add additional sessions.
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