How do I cancel my paid subscription (Wix)?

You can only cancel your paid Socital App subscription from your Wix account. See the Wix help article here.
Note that this is how Wix billing works.

Wix will cancel your paid Socital App subscription at the end of the billing term.
Once your paid Socital App subscription is cancelled, Socital will automatically downgrade your subscription to the free forever plan.