(Encapsulated) PostScript files are normal text files which can in principle be edited directly. PostScript is just another programming language, particularly well adapted to draw lines and arcs on a page. Nevertheless, most PostScript code is nearly unreadable for humans. (And many of them don’t want to learn another language.)
Traditional solution: You have to convert the EPS document to an Xfig document and edit that. Just type the command
pstoedit -f fig figure.eps figure.fig
where the last two arguments stand for the EPS or Postscript and the Xfig file, respectively.
Modern solution: Use a graphics editor capable of opening PostScript files directly. I recommend either Inkscape (free) or Affinity Designer (available in the Mac App Store). You might also want to look at Cenon by vhf interservice.