If you still need 2.5.1, here is the from Netgate’s archive:
pfSense Community Edition (CE) version 2.5.1 was a maintenance release focused on stability and bug fixes following the major 2.5.0 update. While it corrected several high-priority issues, it is now an outdated version, as the current stable release is pfSense CE 2.8.1 . pfsense 251 download iso link
On Windows (PowerShell):