The content of a Welcome Email is configurable for both full and lite users of your instance. To do this go to the admin screen and select Organisation:
On the Organisation screen select Edit, then scroll down to the Notifications section. Here you will see three text fields, one for the Title of the Welcome Email, and one each for the content of a Full User and Lite User Welcome Email:
The content of these fields is what will appear in the Welcome Email above the Sign In button:
Once you are done making changes click on Update at the bottom of the screen to Save.
The subject line of a Welcome Email is always 'Welcome to Folio'. This cannot be changed.