The only solution is to fix whatever is preventing 25H2 from installing. If you can't do that, you'll have to do a clean install of 25H2. Explanation:
1- 24H2 is not publicly available, since it's an outdated version of Windows.
2- Even if you could get your hands on 24H2, that would be the full version of 24H2, not the customized-for-your-computer version that you would receive through Windows Update. Consequently, there's no guarantee that it would install without creating problems for you.
3- The goal of Windows Update is for all Windows computers to be up-to-date. Even if you could install 24H2, Windows Update will try to install 25H2.
Here is the lesson I learned after years of trying to do something else: There are no shortcuts around doing it the right way. And if a shortcut seems to succeed, it will only come back to haunt you in the future ((:-(