need to update particular fields in a nternal table

I had a internal table like

f1 f2 f3 f4

1 1 1 1

2 2 2 2

3 3 3 3

and I want to update this internal table randmly with refrence to the particular field, and remaining fields must be as it is before and my output to be

f1 f2 f3 f4

1 1 [2] 1

2 [1] 2 2

[4] 3 3 3

suggest me in the above please...