When you log in to any affiliate panel, the home dashboard is shown by default and you can see the first message "Welcome to our affiliate program".
If you wish to edit the text of this message you can do so in merchant panel when you navigate to:
Configuration > Affiliate panel > Menus & Screens > click edit on "HOME" (you need to scroll DOWN the list on the right side) > Welcome message field
If you are wondering about the information about affiliate manager, which appears right next to the welcome message, there is a separate guide for that which you can check right here.
Note/Advanced hint: If you are using Post Affiliate Pro or Post Affiliate Network in multiple languages, so your affiliates can choose their desired language, then you are supposed to enclose the 'Welcome message' between ## and ## and add the exact copy of edited 'Welcome message' as a new translation into each imported language.
You can do so by navigating to: Configuration > Languages & Regional settings > click edit (pencil icon) next to the desired language > Translations (tab) > Add custom translation
You can do so by navigating to: Configuration > Languages & Regional settings > click edit (pencil icon) next to the desired language > Translations (tab) > Add custom translation