To remove fields from documents in a collection use:
db.test.update({data:{$exists:true}},{$unset:{data:1}},{multi:1});
Or, if you want to add new fields, use set:
db.test.update({data:{$exists:false}},{$set:{data:"your value"}},{multi:1});
Set “multi” to 1 if you want to update/remove fields in all documents that match the criteria.
Tanya Nam 
Isn’t this removing a single field from multiple documents rather than removing multiple fields from a single document?
Yes correct. I updated my last sentence to be more clear. Thanks for the note!