How to sell Proton
You can cash out your XPR with the same exchange you bought it through:

Sign in to the exchange you have XPR on.
If you store your Proton in a digital wallet, compare crypto exchanges to sell it on.
Place a sell order.
Choose the amount of XPR you'd like to sell.
Complete your transaction.
Confirm the sell price and fees and close your sale of Proton.