Hi,
Thank you for reaching out on the community. It is not possible to flip a texture image inside of Twinmotion at this time, though many image viewing/image editing applications will be able to flip images. When opening the image file on your computer, it is likely the application used to view the file will also have options available to edit it as well.
If you would like this feature to be incorporated in Twinmotion, I would recommend submitting this as an idea on our Twinmotion Public Roadmap. Here, you can provide feedback to our development staff on how you would like to see the feature implemented and it's importance to you.
Best Regards,
Morgan