What exactly is your concern that
a) UPDATE query isn't working? or
b) data values are not changed??
As said previously,
SELECT the records whose values you want to change
UPDATE those records
SELECT the records using the new values
As for run time, what hardware are you using? Is the run time really noticeable?
Put the sequence of queries into a loop process and run.
It's you who is doubting the UPdate, so test your base logic with a few records, make sure it works, then run the loop set up.
It you have additional concerns, then make sure you do a backup before you start --you can always do the process over.
Still not sure what your basic concern is.