Topping/fimming may increase the yield if done correctly in vegging stage. After that needs to be vegged longer. Because plants need to recover after that stress. And new development takes one to two weeks, in total needs to be vegged two months approximately to get some good result.
If they're already in flowering it will not increase the yield. It will just look that way because you'll not have a main cola and energy will be spread to the others and some will be lost by recovering. In the end visually can look good but if you have place to go, go without from now on. If you're limited, you can consider some LST. Simply tie down the main cola from the last 2nd or 3rd inter node. Can be hard if you try to do from the lower inter nodes if the stem is thick. You can keep training for a week or more. Whenever the main bud straights up, tie down again. This usually happens just in a day. When you get most of the colas on same level you can stop training. Check my log, I just did a simple LST on second week of flowering. Can give you an idea.
I previously had two plants, one without training and one with LST. Results looked same to me. I didn't see any noticeable difference except for all visible buds on top with LST. Other without training had the same buds just somewhere in lower place. But they were different strains i must tell.