Remove the concrete tree ring from around the tree by putting the shovel under it and levering it up. Ask someone to help you lift the ring over the tree. If the tree is too tall to lift the concrete ring over, use a sledge hammer to break it into smaller pieces.
Place a peg on the inside of the garden edging, where it overlaps. Use a hammer to drive the plastic peg into the ground. Make sure that the top of the peg's overhanging lip is sitting on the garden edging. Continue hammering in the pegs, so that they're evenly spaced around the garden edging and it is secure.
After hammering in the pegs, use the shovel to backfill the soil into the trench the garden edging is sitting in. Do this on the inside and outside of the edging. This will allow the grass to grow right up to, but not over the edging. Add a healthy layer of mulch around the tree, this will not only help the tree to grow but will also make the project look more professional.