>in vfp i do :
>replace all myfield with recno()
>
>and the command re-number the field.
>
>how can i do it in sqlserver ?

SQL does not have the concept of a record number in the way that VFP does. Why do you want to do this ?
You can assign successive integers to each record but they will be in no particular order:

DECLARE @incrementer int
SET @incrementer=0
UPDATE MyTable SET @incrementer = myfield = @incrementer+1