PC on Insider Preview 26H2 ... Windows Update tries to install 25H2 and fails

Do not run a Windows 11 25H2 in-place repair over build 26300.8935. That setup would be moving to an older platform build, so it is not a supported repair path and may fail before preserving your apps and data. The Unenroll option also does not downgrade Windows immediately: on supported channels it queues leaving at the next public release; other Insider paths require a clean installation. First check Settings > Windows Update > Windows Insider Program and note the channel and enrollment account. Compare the installed build with the latest flight for that channel. If you want to remain an Insider, keep the device enrolled, report the incorrect 25H2 offer in Feedback Hub, and obtain a newer same-channel build or ISO. If you must return to stable Windows now, back up everything and clean-install the current public release. A clean installation removes applications, settings, and files.Do not run a Windows 11 25H2 in-place repair over build 26300.8935. That setup would be moving to an older platform build, so it is not a supported repair path and may fail before preserving your apps and data. The Unenroll option also does not downgrade Windows immediately: on supported channels it queues leaving at the next public release; other Insider paths require a clean installation. First check Settings > Windows Update > Windows Insider Program and note the channel and enrollment account. Compare the installed build with the latest flight for that channel. If you want to remain an Insider, keep the device enrolled, report the incorrect 25H2 offer in Feedback Hub, and obtain a newer same-channel build or ISO. If you must return to stable Windows now, back up everything and clean-install the current public release. A clean installation removes applications, settings, and files.