If you know the PostScript programming language, just use any text editor to edit the code directly.
If you want to do it visually, you can use Canvas on the Macintosh. Arts & Letters software package from Computer Support Corporation is supposed to be able to read arbitrary EPS files, using a software converter called Decipher. Also, Island Graphics has a trio of software packages called, appropriately, Island Draw/Paint/Write. This is also supposed to include a full-blown PostScript interpreter, and be able to read and edit arbitrary PostScript files.