There are two ways to skip upcoming customer payments:
From a customer's profile
Head to Customers >> Customer List >> search customer >> click their 'Memberships' tab >> click the membership name. Then, head to the 'Upcoming Payments' tab, click the payment and then click 'Skip this Payment'.
From upcoming billing
This option is ideal if you need to skip multiple customers' payments at once. Head to Billing/Revenue >> Upcoming Billing.
Here, click 'Options' next to each customer's name, tick 'Skip Payment' and save.