You can remove an air bubble by pricking or poking a small hole in the wallpaper with a needle or the tip of a Stanley knife. Then press the air out of the bubble. With a glue syringe you can, if necessary, inject a little glue into the bubble. The glue syringe causes minimum damage to the wallpaper. For wallpaper surfaces with a uniform colour, a hole may be more noticeable than for example with a bunch of wallpaper.