The proper (and legal) recourse would be to contact the publisher via written letter and explain your intentions. If it's for a festival or competition, they may already have a lesser-known one-act version available on file.   

If not, and if the playwright is still living, the publisher might (and I do mean "might") relay that message to them if such a thing is possible.

The author of the piece is the only one with the authority to alter it. Do not attempt to do so yourself without acquiring permission as you will be opening up your theater, its board members, and yourself to legal repercussions.

Good luck.