Where do I find my Peer ID on a 2820n and how do I change it?

VPN & Remote Access >> LAN to LAN >> ( choose the relevant profile index ) >> scroll down to section 3. Dial-In Settings and there is Specify Remote Peer ID.

If you want to set the peer ID on a dialout profile:
dial-out profile (section2),
IP Security MEthod,
choose advanced,
change to "aggressive mode"
the Peer ID is the "Local ID"