I tried to install Ver. 3.18 x64 this morning and although it went through the full process of doing so, it wasn't there after clicking 'finish' on the installer and 3.17 was still installed. No errors were reported.
My system is: Windows 11 Ver. 21H2, 16Gb RAM, 68Gb free space on the C:\ drive
I can confirm I do have two versions on my system. I have discovered that version 3.18 is installed but there are no startup menu entries or shortcuts to it. Those belong to version 3.17 although there is no uninstaller for 3.17. Can it be uninstalled leaving 3.18 intact?
1.) Start the old existing v3.17 Mp3tag manually
2.) Create a backup from your v3.17 configuration, according to the mentioned notes from inside Mp3tag v3.17 and File -> Save configuration
3.) Re-Install v3.17 32bit-Mp3tag "over" your existing v3.17 32bit-Mp3tag -> this should create a deinstallation entry in your control panel and Start menu
4.) De-Install v3.17 32bit-Mp3tag
5.) Re-Install v3.18 64bit-Mp3tag
Save and backup all your configurations first as described in the linked. extra notes.
Then un-install both versions.
If one does not disappear, delete it manually from the folders.
Then install the new version and restore the configurations from the backup.