Halo: Reach is the first part of the Master Chief Collection that became available on PC and, although you can get it on Steam, you might need to change your Xbox Live gamertag.
Whether you can't remember your old Xbox Live gamertag or are just now setting up an Xbox Live account, here's everything you need to know in order to start playing Halo: Reach
Halo: Reach Steam How to Change Your Xbox Live Gamertag
Even if you own Halo: Reach or The Master Chief Collection on Steam, you'll still need a Microsoft account in order to log in and play. If you're unearthing an account you haven't used in a while changing your Xbox Live gamertag might also be required.
The catch is that you can't really change your Xbox Live gamertag in Halo: Reach itself. Instead, you'll need to download the Xbox Companion App from the Microsoft Store or from Microsoft's website.
Upon launching, simply log in using the same account you used for Halo: Reach and click the user icon on the left side to open the settings. You should, then, easily spot a Change gamertag option. Clicking it will take you to the Xbox website where you'll be able to fully go through with the action.
Once you've changed your Xbox Live gamertag, you should be ready to hop into Halo: Reach using your account.
Halo: Reach is now also available on PC and a lot of people have been waiting for it, at least judging by the Steam Player count.