From my experience, be carefull what you do. I also searched many different aproaches on hot to relax my upper lip. Once i did find some streching video on youtube,so I tried that.The next day I had rehearsal and I couldn’t play above C2 ( french horn ) which is the next to middle c . My lips just didn’t want vibrate in upper register ,at all! So ,for me, no more streching of the lip. But maybe I overdone it a little ,hehehe . 🙂
If you want,you can massage your face with slow movement.
I think you can try Tanaka face masage https://www.youtube.com/watch?v=LrceQS7qdaI&list=PLqP0GGk9REz_47GJ6os5STEQsFFeSGSGE (at your own risk ,of course)