First Release or Early Adopter as I would Call it, is Nice for beeing in the front and test feature before others.
To enable First Release in Your tenant:
1. Have Global Administrator rights for Your tenant.
2. Go to the Office 365 Admin Center
3. Go to Updates and turn on First Release.
Notice that this is a tenant setting, so all users of Your tenant will be receiving these updates.
For some organizations that would not be the best way to introduce New features, so I wouldn’t recommend everyone to turn on First Release. You’ll might end up With a lot of phone Calls if the update is within the users eyes and they don’t know what it is, or if the update acctually changes something for the users.
Would be Nice to have an First Release for certain users, for example the Global Administrators. Allthough I do understand some updates are tenant wide, and can’t be hidden from some users.
Have a look at this to understand how updates are released:
First announcement is on the Office Blog, then the Message Center in Our Office 365 Admin Center. Atleast 7 days before First Release enabled customers will have it, and from the same point atleast 21 days before Standard Release. Depending on the update it can take up to a year before every tenant have received the update.
For bigger updates like Wave 14 to Wave 15, there will be more information and time. Especially for SharePoint Online to make customers ready for the Upgrade.
Source: Office blog.