In order to clear your mapping of existing data and start afresh: go to Items -> Manage Items -> Import / Export tab, click on Import and select your Excel file to import.
Then click on Reset. This will reset the mapping of the fields, and start the uploading wizard screens from the beginning where you can indicate wether or not you will be using Matrix items.
Then click OK
Click Yes or No regarding the matrix:
If you selected "Yes" for Matrix it will also ask you if you require a 1 or 2 dimensional matrix. 1-dimension is for one variant only (for example size only) and 2-dimension is for two variants (for example color and size).
Select your currency and then click Upload File.
Go through the tabs, selecting the column required:
When arriving at the last step select Advanced Options to get back to the typical mapping screen:
You will now be brought back to the typical mapping screen. All previous data and mapping have been removed, and you are starting clean. Now continue by mapping your Excel file: