how do I sort my bookmarks alphabetically in Opera 26
Drag and drop, unfortunately.
You have to wait for the next version (I don't remember if in version 27 or 28 there's the possibility to sort bookmarks)
Or, in the current version, you can temporarly move the bookmarks in the bookmarks bar where there's the option to sort the bookmarks and then move them back in bookmarks
So long as you have the bookmarks manager CLOSED, you can navigate to the folder you want to alphabetize using the "Bookmarks" menu item fourth from the top on the O menu, right-click the folder, and select "Sort Alphabetically." This will not work if the bookmarks manager is open.