Been almost 4 decades since I've repaired a plastic bumper, and zero experience with the Sub plastics, but it is almost assuredly a thermoplastic, so it will soften with heat as eagleeye stated. Try at first to push it out without any heat, but if that takes too much effort, proceed carefully with the heating. Heat it slowly to maybe 150 degrees ( F, not C! ) while applying pressure. Watch carefully that it does not try to crease someplace, as that will both ruin the paint AND the plastic in that area. Depending on the plastic, anything above 200 degrees may soften it so much that you can inadvertently stretch it, creating an outwards bump, so proceed very slowly with only enough heat to soften it enough to move.
I would guess that you will want to first apply pressure on the area right below the top crease to slowly straighten the crease without causing an outward dent that could happen it you simply push at the center point.
Use a heat gun and keep it moving. Best done with the bumper cover removed so as to not burn/bubble the paint, and use the heat gun on the BACK side of the cover.
Move the heat gun around constantly ensuring an even heat. If you start to see a discoloring of the plastic, remove the gun. When warm enough to reshape, rub the round handle end of a plastic screwdriver into the backside of the dent and move/smooth it around with moderate pressure to force the cover back into shape. It may take several operations with the heat gun and the screwdriver to get things back to near normal shape.
The key is to not get the plastic too hot. You might practice on an old bumper cover section obtained from a wrecking yard first, before you do yours, because there is a fine line between not enough heat and too much.
We do these kind of repairs all of the time in the shop. Most times the results are fantastic. Good Luck!
I can confirm that the procedure outlined by @gbhrps works, as I once had a body shop successfully push out a bubble in the corner of my Outback's rear bumper, with absolutely no leftover visible effects.
However, the bubble did not have a "crease" in the plastic to start out, the paint on the plastic surface had not cracked, and there wasn't a metal trim strip in the middle of it that had dented. I see all of these issues present in the picture provided by the original poster. And yet, in my case the shop still required my car to be left all day, and in their words their plastic specialist was very gently heating it and going at it from the backside about once an hour with some shaping anvils in order to very gently coax it out without leaving visible traces of his work. This told me it was a pretty specialized procedure and not for the average DIY'er.
It was well worth the price they charged (~$100 IIRC) because it saved replacing and repainting the entire bumper cover.