How to add extra new term into multiple nodes I got 100 nodes. They all have Term reference field. 50 nodes are set to term 'A' and other 50 to term 'B'. So with that it helps me to filter those nodes via Views to display A nodes in one page and display B nodes in other page.

So now there's an update needed to be done. All those 100 nodes has to have a new term selected 'C' (A and B stays selected as it is). So all A's has to have now C and same with B's.

So how do I do that instead of going through all 100 nodes single by single and selecting it manually?
Install the Views Bulk Operations module.    
Create a view and add a filter to view nodes having term a and b.
add    a Bulk operations: Content field and in the field  settings under    SELECTED BULK OPERATIONS select  Modify entity values.
Save the view and from the Operations list, select Modify Entity
Values.    Choose the term field you want to add the terms to,
and then    select the terms you want to add
Select the Add new
value(s) to    FIELD, instead of overwriting the existing values
check box and Submit the form.