You cannot get a replacement naturalization certificate fully online
File Form N-565, Application for Replacement Naturalization/Citizenship Document, with USCIS
Submit the form online through a USCIS online account if eligible
Upload required documents and supporting evidence in your USCIS account
Pay the filing fee online
If you are not eligible to file online, mail the paper Form N-565 to USCIS
Use your USCIS receipt notice to track your case online
Attend any USCIS biometrics appointment if requested
Receive the replacement certificate by mail after approval
If you need proof of citizenship sooner, request a USCIS case status update or InfoPass appointment if available