To create automatic messages for your customers you need to create follow-up points. See the documentation here to get started and find example templates.
To change an already created follow-up point, go to Configuration > Marketing activities > Follow-up points.
Note! Follow-up points can only be configured to be sent out at a specific time according to the parameters set up for that specific follow-up point. If you want to edit the automated message that is sent out from a trigger (e.g., a subscription being bought) then you should instead edit the message templates: