How to delete the primary index of a table in Progress, release 11.6?

I have made a table, containing an integer field, this field being used in the primary index of that table. Now I would like to replace this integer field by a character one. Therefore I need to remove all indexes of that table, including the primary one. Therefore I'm using the data dictionary, but I get following error message: You cannot delete the primary index of a table. How can I proceed? Thanks