Step 1: use a find to find all the records with 'x'.
Step 2: use Replace field contents to update the fields with a 'y' in place of your 'x'.
Read up on Replace Field Contents in the on line help to understand all the different options.
Experiment with a copy of your file if you aren't absolutely sure you know what's going to happen as this tool can change a lot of data in one step.
Don't do this on a networked DB if there is any chance that another user could be editing one of the records. Take it down off the server or do it after hours.