Hi,
Thank you for reaching out to us.
To change “Purchase & earn” message, go to WPLoyalty -> Campaigns -> choose the created Point for Purchase campaign -> Advanced Settings -> Message for simple and variable products (from here change the message and Save the changes).
Here is the screenshots which might be of helpful:
https://nimb.ws/lyTfdZ
https://nimb.ws/1evfKI
I hope that helps.
Thanks
Team WPLoyalty