Unfortunately the template for sharing your app is not changeable but you can change the message on the fly if you'd like. Follow these steps to change the message which is included when sharing via text message for example:
1. Click "share app" from the admin section and choose text message
2. Your text message window will pop up with the template and link to your app
3. Change the text which is showing up under your link
4. Hit Send
It is on our feature road-map to make these templates changeable so you do not have to change the message every time if you want different text to show up.