As I played through the game I unlocked more options, one of which being to change my stamp from the OK to whatever I wanted to draw. I imagine that, like that option, the option for StreetPass message will unlock at a later date. Alternatively, it may unlock after you StreetPass for the first time? But it's probably a later-date unlock.
Give it a few more days and you will get another chance to change your streetpass msg. Then make sure you write your own message. This option to change your message comes out every so now and then. About once a week for me. Other than that, I don't think there's a way you can change it manually.