EDIT QUICK RESPONSES
The watch lets you send a couple of set responses when you receive messages and texts. The default messages are generally in the tune of Yes, No, Thanks, etc.
If you are used to sending slightly longer responses, then you might want to add new ones. For now, the system lets you add 30 characters in one message. So, you will have to be creative about what to add.
To add new Quick Responses, go to Watch settings > General > Quick Responses, on your phone. Next, scroll down the page until you see the button for Add response.
Source : guidingtech.com