Here's how I "cheat" on that.
Make a copy of the data in question. (NEVER EVER work with the production file NOR the only copy of the data!!!!)
Now, using the standard Query editor, build you select query to return the records you want to update. Run this and make sure that the records you want for a single instance are returned.
Convert to the update query.
enter your update values, etc...
save
run
double check that this updated the desired records.
Switch to SQL view
And now you have the basic SQL script.
You can use parameters at this point or you can use the form and pull that information from the form and build a SQL to use with the execute method.
I'm going to send you to MS for a tutorial for how to:
Use parameters in queries and reports
You should be able to modify the saved query.
Once you have this working in the copy of your database... simply import it into the production database!