Update a table while reading it

Question

I have a database table with a Y/N flag in one column. I want to read all records where the flag is 'N' and after processing a record, set the flag to 'Y' in that record. Is it correct, and reasonable, to do this at the same time, using two separate connections? Or should I read the entire table first and update only after I'm done with the reading? What's the correct approach to this?

The database involved is Netezza, in case it matters.