Ensure iexplore.exe is not running in Task Manager. The ActiveX control cannot be overwritten if Internet Explorer is open.
There are several "abandonware" websites that host legacy software installers. Sites like OldVersion.com often host clean installers. When downloading, look for filenames similar to: adobe flash player 11 activex 64-bit free download
The most reputable source for finding a legitimate installer is the Internet Archive (archive.org). Tech enthusiasts have preserved direct copies of the original Adobe installers here. You can search for "Flash Player 11.9 ActiveX" (the last iteration of version 11) within the archive. Ensure iexplore