If you're on the Windows Insider Program and you're getting a message that the build you're on is about to expire, you're not alone. Due to an oversight, the build expiration date was set to July 31 all the way up until build 20161, which was released on July 1. Two weeks later, build 20170 was released, changing the expiration date to January 31 of next year.
Unfortunately, build 20170 has an issue with AMD processors, so if you're like the user linked to above, you're still stuck on build 20161. The good news is that that bug should be resolved, and you should get a new build long before the expiration date. Builds usually arrive on Wednesdays, so assuming that there is one tomorrow, you'll still have nine days before your build expires.
There's another group of users that are going to have issues though, and this has to do with how the Windows Insider Program has been restructured. Previously, the Fast and the Slow rings have been on the same development branch. Now, the Dev channel is in a perpetual state of prerelease builds while the Beta channel is tied to a specific release.
When switching from the Fast ring to the Slow ring, or switching from the Dev channel to the Beta channel, the process has been the same for a while. You'll stop receiving builds, and you'll get a new update whenever your channel catches up. This worked fine in the Fast and Slow ring days because they were getting similar builds. It's different now, as the Beta channel is on build 19042 and the Dev channel is on 20xxx.
In short, if you switched your machine from the Dev channel to the Beta channel before build 20170 came out, your build is going to expire on July 31. Your options are the same as they've always been. You can temporarily switch back to the Dev channel to get a new build, which will expire next January, or you can do a clean installation of Windows 10.
You can, of course, wait for an update. To be clear, expired builds won't put your machine in an unusable state like they did years ago. You'll just keep getting warnings about it, and then those warnings will get more frequent. The problem with waiting for an update is that you're going to be waiting for a while. It seems that 21H1 isn't going to be the major update that we were expecting, so if you thought the Beta channel would catch up in September or October, you're probably wrong.
Now that the Windows Insider Program has been completely restructured, albeit gradually over the last year and a half, this is a problem that Microsoft is going to have to sort out going forward. Build expiration dates typically changed every six months or so, and the Dev channel switches to the Beta channel yearly at best.