This might be different for the M8, but on my phone I go to the phone app > settings > personal settings > quick response and I can edit the preset messages.
Edit: I was just told by someone with an M8 that you can't edit the preset messages in the phone app's settings.
On my Evo LTE the dismissed call reply messages were the bottom 4/5 on the quick text list. I haven't tested this yet, as I've only had the M8 for 2 days.