You can launch regedit and delete the Opera uninstall key for the older entry. For current-user installs, the Opera uninstall keys will be under "HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Uninstall". For all-user installs, they will be under "HKEY_LOCAL_MACHINE\SOFTWARE\WOW6432Node\Microsoft\Windows\CurrentVersion\Uninstall". You can goto the URL opera://about and it will tell you what install type it is. To delete a key, you right-click it and choose "Delete", but you have to make sure you right-click the right Opera key.
Then, if Windows settings are open, you just have to close them and open up the "Installed Apps" settings again to see the changes.
The safer way though is to do as said and uninstall (but don't choose to delete your data when asked) and reinstall. It's up to you.