You cannot change Insider program settings in build 14905, fix coming soon
There were many known issues written in the blog post, but there is an issue that wasn’t written and was discovered later. This issue prevents the user from changing settings on the windows Insider program in Settings app.
Jason, Program manager of windows Insider program has posted some information here, a part of which is mentioned here:
This bug only affects users on build 14905 for PC. It *does not* impact Mobile users. Users on this new build will not be able to change the Insider Preview Build settings on their PC. The cause of this issue has already been discovered and a fix is already in the works. This fix will be included in an upcoming flight.
There are no workarounds for this issue. It does not impact core OS functionality, only the ability to change flight settings. If you find it necessary to make changes before a future build is released with the fix, you may roll back to your previous build via the Settings menu. This however should not be required.
Expect these kind of bugs till the builds from the development branch continues, after all Fast ring is for the people who can bear with bugs.