Connect a computer to the LAN port of your pfSense firewall. Your computer should be configured to obtain an IP address automatically via DHCP from the pfSense server. Once connected, open a web browser and navigate to the LAN IP address you configured during installation (typically https://192.168.1.1). Accept the self-signed SSL certificate warning. Log in using the default credentials: username "admin" and password "pfsense".
Getting a specific, older version of pfSense like requires a bit of a workaround since the official download page usually only features the latest stable release (currently 2.7.x). Pfsense 2.5.1 Download Iso
To find the specific 2.5.1 ISO, you have two primary options: Connect a computer to the LAN port of your pfSense firewall