How to mass change a member attribute for several people?

Go to Reports > Members, select Mass Update Member Attribute. 

Select the Member Attribute you want to update from the Options column on the left.  Then assign the updated value to this Attribute.  See the example below for mass updating Church Member Type to reassign Guests to Member. 

  Use the Filters column to filter to those whom you want to update.  In the example above, we selected the Members filter bar, then filtered to existing Guests.  

Click Run Report to preview the selected individuals to be updated.

Click "Update" and the change will instantly be made.