See this link for instructions on How do I amend a 2018 return in TurboTax?
It would be very unusual for you to have recently received a 2018 1099-K from PayPal. Assuming that is the case, the 1099-K is typically reported as "General Income" (cash, checks, credit card) on a Schedule C (self-employment) or Schedule E (rental income). Both are found in the Business interviews of TurboTax Self-Employed (or Home and Business).
