Unix Shell Scripting to Change the header of the file in Unix

Hi,

I have a situation here:

I have some files:
 update_1.dat, update_2.dat,update_3.dat.....update_x.dat

Each file will have a header as first record in it to determine it's sequence.

Say for ex:

update_1.dat
001|201501300940193201|XXXX02|1|201501303|YYYY30|20|1|1|

update_2.dat
001|201501300940193201|XXXX02|1|201501303|YYYY30|20|1|1|

The above is what I have right now..

I am expecting and what I am trying to achieve is
001|201501300940193201|XXXX02|1|201501303|YYYY30|20|1|2| -- Update_1.dat
001|201501300940193201|XXXX02|1|201501303|YYYY30|20|2|2| -- Update_2.dat and so on

If I have three files ,then  header should be

001|201501300940193201|XXXX02|1|201501303|YYYY30|20|1|3| -- Update_1.dat
001|201501300940193201|XXXX02|1|201501303|YYYY30|20|2|3| -- Update_2.dat and so on
001|201501300940193201|XXXX02|1|201501303|YYYY30|20|3|3| -- Update_2.dat and so on

I want some advices and suggestions to achieve this.

Also, it would be great if I get the sample code piece to achieve the above scenario.

Thanks