Thanks for reaching out to TextNow Wireless support. Our development team is dedicated to improving our devices by bringing the latest software to them. With that in mind we want to roll out the next update as smooth as possible, to heighten the user experience. While we do not have a time line for the Marshmallow update rest assured we are working on bringing this update to our phones in the future.
We do not recommend flashing your TextNow device because it would remove the VoIP network optimizations built into our custom OS. It would also void the 1 year warranty on the device if anything was to go wrong.
If you have any questions or concerns let us know and we will assist as best we can.