You file as statement to revoke S-corp status (which makes you a C corp) then file Form 8832 to change from C to disregarded entity.

I don't think there is any late relief for these elections, which kind of sucks because the due date for Jan 1 2019 was March 15, 2019

https://www.irs.gov/forms-pubs/revoking-a-subchapter-s-election

https://info.legalzoom.com/terminate-scorp-election-revert-llc-23367.html