If you purchased a digital version of one of the games (prior to May 25th, 2018, the end of the Myst 25th Anniversary Kickstarter campaign) through GOG.com or Steam, your existing game will be updated if you have "automatically keep my games updated" enabled on your account. 

Otherwise, you should be prompted to update the game when you log in to the Steam or GOG account your games were redeemed to.