Many users encounter this issue while updating , inserting or deleting data from views .

Following is the link from MSDN which can clear the background while encountering the issue :

https://technet.microsoft.com/en-us/library/ms187956.aspx

Please read the updatable Views and Partitioned Views section in the above link .

For Instance : You will not be able to update the below View because a  View whose columns are a result of set operators like union , union all , intersect from other views or tables .

Create VIEW
[myview]

as

select a,b,c from X

union

select a,b,c from Y

Thanks