How do I get rid of bubbles which evolved in a wallpaper?

Bubbles are usually air pockets or a small bump on the wall or even debris that were overlooked when the paper was hung. Press on the area and if you feel a speck of something cut an X through the paper, remove the debris ,then, apply a small amount of adhesive on the wall area and roll the paper flat. If the bubble is in fact just air, you can fix the problem with a glue-injecting syringe,  fill the syringe with some adhesive, cut a straight small slit with a razor knife and squirt in the adhesive, then flatten the repair and wipe off any excess adhesive.

Use a pointed object to remove the air inside your wallpaper needles would be advisable larger than that would ruined your wallpaper.

try making a small knick in the center of the bubble to remove the air.  That is my best guess.  It works for me on shelving liner.