SQL Express - Identity specification property - how to change

Hi, I use SQL Server Enterprise Manager for MS SQL Express and i want
to change the column properites for [int]
type of column, but I cann't change it.

More in details...
I'd like to set autoincremntal option and in order to change that I am
trying to change Identity specification from no to yes, without
success.
It is primary key too,
Name AricleID,
int type,
Primary set of binding --> ((0))
Full text specification --> No
Identity specification --> No --> I want to change this

Everything else is disabled

Thanks
.