Go to your Grow Overview page and locate the plants in your "Veg" column that you would like to undo the creation of, clicking on the strain name of those plants.
A window will populate titled "Vegging Plants." In this window, first make sure you select all the plants you want to undo the creation of.
Once you're sure you've selected all the plants you want to undo the creation of, click on the orange rectangular "Undo Plant Create" button in the top left corner of the window.
A window will pop-up asking if you're about making this change. If you are, go ahead and press that OK button!
Now you should see all those "plants" return to where they came from in your "Clones, Seeds" column! Nice work!