Add a / in the middle of string

I need to be able to insert a / in the middle of a string, so this 125674567 ends up like this 12/567/4567.

I am using SQL Server 2005.
I have used the SUBSTRING Function to display this, as below:
SELECT
    SUBSTRING(Holding, 1, 2) + '/' +
    SUBSTRING(Holding, 3, 3) + '/' +
    SUBSTRING(Holding, 6, 11)
      FROM TagOrder_Import_RawData

How would I be able to update the records in the table?

Any help would be appreciated
Thnaks