The metal of your car expands when it gets hot so sometimes parking your car in the hot sunlight will encourage tiny dents to pop back out. Remember, this fix is not instant. It depends on the weather and the length of time the car sits in the sunlight. Ideally, leave your car a few hours in the sunshine to encourage the dents to hail damage to pop out. This is unlikely to work and if it does, it would only be smaller dents that are already nearly invisible.
Just like heat will pop out the hail damage the other extreme also helps. Don a pair of gloves and apply dry ice to the area of worst damage. The sudden cooling of the dry ice on the metal causes the dent to spring out. Again, this can absolutely work but results are mixed and depends on a number of factors.
Dent pullers come in a few shapes and sizes and can be purchased at an auto parts stores or online. The dent puller itself is a mechanical device which sits on the car and grabs a small puck which is hot glued to the dent. There are a number of different shapes and sizes of puck and the person doing the repair has to have a “feel” for when to use the right one. You essentially glue them to the car then use the dent puller to bend the metal back. One big pull will yank the glue right off the vehicle surface without damaging the paint or leaving residue behind (if done right). This is a solution that many “paintless dent repair” outfits use to fix dings and dents from hail or otherwise without requiring a costly repaint.