Right-click on a Text field and select Replace... in a context menu.
The Replace dialog displays.
Select By Position and Length.
Enter the value or click in Start Position to define the where the replacement will start.
Enter the value or click in Length to define the length of replacement.
Enter the value or click in Sample Row to select the row number that a sample of the replacement definition that will be displayed in the Result Column box below:
Check the Remove Original Column box to delete the original column and just display the new column with replaced values instead.
Click OK. The new replaced values are displayed.