Unfortunately once a membership is activated we cannot simply change the activation date of the membership itself, however there is the ability move future scheduled payments and to alter the validity of session credits to align with the new desired "Membership Start Date". (Please note: scheduled payments can only be moved forward and not backward. If a payment is required prior to the first listed 'future payment' than an entirely new payment schedule needs to be added by clicking the + icon). Alternatively, if you would like the membership start date to be reflected on the membership, the easiest option would be to cancel the membership and reallocate a new one.
Follow the below step by step guide on how to amend/update the start of a membership: