You can delete all the version folders you don't want.
In the version folder you keep, delete opera_autoupdate.exe so it doesn't update on you. Just note that this will break checking of updates and you'll have to monitor https://blogs.opera.com/desktop/ for new versions and manually download updates when you want to upgrade. Then, if you do upgrade, you'll have to delete opera_autoupdate.exe again for the new version.
To get launcher.exe to use the version you keep, open installation_status.json in a text editor and replace all occurences of the version number you see in there with the version of the folder you kept.
Then, you're good to go and you can delete the launcher.old files. If launcher.exe fails to load Opera for some reason, you'll have to rename it and then rename one of launcher.old files to launcher.exe to find the one that works. Then, you can delete the other launchers.
But, it'd just be easier to uninstall Opera while choosing to keep your data., delete Opera's install folder, download the version you want from https://ftp.opera.com/pub/opera/desktop/ and install it. Then, as soon as that Opera starts, close it and kill the Opera autoupdate process in the task manager. Then, goto the version folder in the install folder and delete opera_autoupdate.exe so Opera doesn't update on you.
But, you can do it whatever way you want.
Of course, it's doubtful that doing that will restore your opened tabs.