This article is from the Miscellaneous Macintosh FAQ, by Elliotte Rusty Harold email@example.com with numerous contributions by others.
In the most basic sense PostScript files are just ASCII text, so
if you're familiar with the PostScript programming language you can
edit PostScript in any good text editor. However if you want to edit
the PostScript files graphically, you need Adobe Illustrator 5.5 or
later. Use the bundled Acrobat Distiller to turn the PostScript file
into a PDF file which Illustrator can import and edit. If the file
includes embedded EPS bitmap images you may also need Photoshop or
another paint program to edit those.