It's the first item in the "File Operations" section of the menu list
... and then the Comparator box opens.
You should see two side by side images.
Right click and select LOCK Images
Depending on your mouse type, the center wheel will change magnification, or Right Click and zoom out.
Now simply check the green check mark of the better of the two, The other image will be replaced with the next one of your group of selected images....
.... and you keep checking the better of the two until there's only one left, and it says, CONGRATULATIONS
When you exit, the one you deemed BEST will have a selected thumb.
Fred