Jose,
I guess your question was recently questionned and answered in this list. I sugest you look for past emails in the list to see if the a possible solutions is waiting for you elsewhere.
Simple way to do that is open you original rec file to enter data as usual.
Find the record you want to update. (this could be done by a autoserach or key unique or Ctrl+F)
Press F4 and a field with which variable you want the cursor to go will appear. e.g. fill with v85
Update the fields u want then press Ctrl+N (or F7 or F8) and save the updated record.
This is the easiest way I can imagne, but you will see all fields in the file, not only those fields you will update.
A more complicated way is to export the file to epidata format filtering the variables you want with the unique identifier of each record: say chart number and v85 to v91 (use option 6 of the numbered menu, it is very intuitive)
Update the file as usual.
Merge (last option of the third EpiData menu from left to right) your new file with the old one side by side, with the new one updating the old - this is also very intuitive in EpiData.
Good luck, Pedro Brasil