How do I make the peer update a global variable?

I know the host broadcasts the "message" ( Global Variable) to the peer, but say, how would I have the peer update the Host with his global variable?

i.e., On clicked, add 1 to Rematch(Global Variable). If Rematch is = 2, then do stuff.

Currently I cant wrap my head around where to broadcast the "rematch" message. Each side just puts there own Global Variable to 1, rather then syncing.

I have searched forums/google, and looked through the multiplayer tutorials, yet I still cant find anyway for the peer to add his value to the variable and sync it with the host. Any help is greatly appreciated, thank you!