Setup Site to Site VPN at Fortinet Firewall

Hi... We are having a 3 site with fortinet 60B and with Site to Site VPN setup. I am replacing one of the units with new Fortinet 60E. I have managed to set up all the basic configuration but not able to get the VPN configured. Our setup is very simple and I can show our existing setup for the VPN setting for your reference to set up the new unit. We need a network administrator expert familiar with Fortinet firewall device.