How do you remove bubble in a linoleum floor covering?

That "Bubble" is more than likely an area where moisture has migrated up and condensed under the covering to form water. This water is harmful to the glue or mastic that was used to stick the linoleum to the substrate.

You can use a razor knife held at a 45 degree angle to slice open the center of the bubble. An absorbent cloth will absorb moisture underneath and a hair dryer to evaporate any remaining moisture will restore a dry surface. Apply glue starting at the outer edge of the bubble working your way to the center where your cut was made. Pressure Roll toward the center and see glue coming up through the angled cut. Remove any excess glue that comes to the surface from the cut and place something flat with a weight on top and allow to cure for the recommended time.

Any floor covering is susceptible for moisture to collect underneath and is especially true in rooms that have water in them. [Baths, Kitchens and Laundry Rooms] If the substrate is concrete, All Concretes Should Be Waterproofed before any floor covering is installed. That will stop moisture vapor emissions that cause this.