In Beta, click the account icon on the address bar and log into your Opera account. Then, click the icon again and click "Sync" to goto the URL opera://settings/syncSetup. There, choose to sync everything (don't choose to create a custom passphrase). Then, close the syncSetup tab to initiate syncing. You can then monitor the syncing at the URL opera://sync-internals and even trigger updates if needed.
Then, goto https://www.sync.opera.com/, log in and confirm your data is there. Then, you can close Beta.
Then, in Opera Stable, log in to your account, goto the syncSetup page, enable syncing of everything, close the syncSetup page and wait for things to come in. Goto opera://sync-internals if needed. After a while, close Opera and open it back up if you have to.
If you need to retry syncing of Opera Stable, you can close it, and delete the "Opera Stable" folders in both "C:\Users\yourusername\AppData\Roaming\Opera Software" and "C:\Users\yourusername\AppData\Local\Opera Software". Then, try syncing again.
Or, for a way to do things without Sync, close both Opera Stable and Opera Beta. Then, in both "C:\Users\yourusername\AppData\Roaming\Opera Software" and "C:\Users\yourusername\AppData\Local\Opera Software", delete the "Opera Stable" folder and rename the "Opera Next" folder to "Opera Stable". Or, you can make a copy of "Opera Next" and rename that copy to "Opera Stable" so that Beta still has its folders if you want. It's up to you. This method usually works, but there can always be bugs caused by the profile being run with different Opera versions, so it's good to have backups if you have anything important.
Another way you can do things is via export and import. At the URL opera://password-manager/settings, you can export your passwords in one Opera and import them in another. For bookmarks, you can export your bookmarks at the URL opera://bookmarks and import them by going to the URL opera://settings/importData and choosing "Bookmarks HTML File" in the drop-down. For open tabs, before your export your bookmarks, right-click a tab, goto "save" and choose "all tabs to speed dial folder". Then, repeat for each workspace and window if needed. Then, after importing bookmarks into Opera Stable, you can goto the URL opera://bookmarks, right-click on a folder for open tabs and choose "open all".