That version is the newest version at this very moment, so there shouldn't be an update.
But, if you want, goto the URL opera://about, take note of the "install" path, download "Opera_120.0.5543.161_Setup_x64.exe" from https://ftp.opera.com/pub/opera/desktop/120.0.5543.161/win/ and close Opera.
Then, uninstall Opera but don't choose to delete your data when uninstalling.
Then, delete the install folder.
Then, delete everything in "C:\Users\yourusername\AppData\Local\temp" and "C:\Windows\temp".
Then, launch the installer you downloaded, click "options", make sure everything is how you like it and install.
Also note that if you goto opera://update or opera://about as soon as you start Opera (before the updater process gets a chance to load), checking for updates might fail. So, always reference the page and check for updates multiple times to be sure. Or, check the ftp site to see if there's a newer update. Link above shows you where regular Opera installers live. https://ftp.opera.com/ftp/pub/opera_gx is where Opera GX installers are at.
Also, it might be good to check if your computer can even connect to Opera's update servers. You can open up a command prompt and try:
ping autoupdate.opera.com
ping autoupdate.geo.opera.com
ping us-autoupdate.opera.com
You can also check opera.com in general if you want.
You can also check the status of the autoupdate server at https://status.opera.com/.
