You need to provide more information if you want specific help, but it is quite easy to load a qvd (all the fields except that one that needs to change) and the new value for the field that needs to change:
'New value' as field3,
FROM MyDataFile.qvd (qvd);
STORE Data into MyDataFile.qvd (qvd);
I suggest that you save a backup of the qvd before running this script.