Changing results in ALV internal table

Hi experts,

I'm new at programming in ABAP and I really hope you could help me with the following issue:

I have the report 'XYZ', which shows an ALV, filled with internal table gtab and at the same time,

the gtab is filled with information from internal table itab1 and itab2.

Now, the gtab is something like this:

Question is: how can i change the value from "serial begin" when its level 10 (the one with "001") with the value from serial begin level 20 (076).

I mean, output should be something like this:

Thanks in advance.

++Kleight+/+