You shouldn’t use a at-home COVID diagnostic test that’s expired, but some have been extended by the US Food and Drug Administration. If your box of test kits shows a past expiration date, check this FDA list of extended expiration dates to see if it’s OK to use yours: 

https://www.fda.gov/medical-devices/coronavirus-covid-19-and-medical-devices/home-otc-covid-19-diagnostic-tests