If your Exeggutor's Hidden Power is Rock type I'm afraid there's no way to change it's type without hacking.
Hidden Power can be all types except Normal and Fairy (despite being classed as a Normal type move). Hidden Power can't be Fairy because Hidden Power was released in Generation II and the mechanics have stayed the same since then, Fairy type was released in Generation VI.
The type and power (Gen II to Gen V) of Hidden Power varies based on a Pokémon's IV. Since you can't change your Exeggutor's IV's other than by hacking you can't legitimately change your Exeggutor's Hidden Power.
If you want your Exeggutor to have a different Hidden Power type then I suggest getting another Exeggutor sorry.
Hope I helped :)