PS this is a self note!
I use to creat a working area to store the data temp and then only modify the Internal structure which is quite a hassle.
Short cut!

* Declare the type of Internal structure!
TYPES: BEGIN OF ty_report,
         SKIP(1)      TYPE C,
         SEQNO        TYPE I,
         butxt        TYPE t001-BUtxt,      " Company name
         pernr        TYPE p0761-pernr,     " PF No.
       END OF ty_report. 
* Declare the Internal table Please take note of the declaration which will help us modify the table direct
DATA: t_report TYPE STANDARD TABLE OF TY_report WITH HEADER LINE INITIAL SIZE 0. 

* the coding below is when you want to change it
    LOOP AT t_report  WHERE SKIP NE 'X'.
        t_report-butxt = 'TESTING'.
        MODIFY t_report.
    ENDLOOP. 

 
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simple eh phew...