The -hflip and -vflip operations flip the contents of the chosen pages horizontally or vertically. No account is taken of the current page rotation when considering what ”horizontally” and ”vertically” mean, so you may like to use -upright first.
cpdf -hflip in.pdf even -o out.pdf
Flip the even pages in in.pdf horizontally.
cpdf -vflip in.pdf -o out.pdf
Flip all the pages in in.pdf vertically.