When you import a file as a Section into Hyperion, it maintains a link to the existing file for the exact path and file name. When that link is broken and that section gets a Process or Refresh command, is the only time it will prompt you for a new file. Otherwise, it will refresh the data from the existing.
So, if you want to force it to prompt you for a new file, you have to move or rename the old file.
But, you're looking for the Pivot and Computed columns to refresh. Two things on that ...
Computed Columns: You don't have to re-import the file, if the rest of your data is current. Each column can be refreshed individually by right-click, Modify, then clicking Ok. You don't have to change any of the code; just hit okay instead of Cancel.
Pivot: In the menu, you have to set your pivot options appropriately to update when you Process (when the underlying data is Processed, really), or Manually (when you Process that section).