As far as I know, you always have to do that in the explore.garmin.com website. For me in the US, after login I select the MESSAGES tab and then there is a section of the page about Check-In Messages with an "Edit" button. Editing the contacts for one of the three canned messages will change it for all three.
hmm, the second one, editing checkin with the messenger app might be so in the manual, but current versions of messenger app have no more way to edit anythnig
