You inflate a balloon with a known volume v₁ of air at ground level and you use a barometer to tell you the atmospheric air pressure p₁. You take the balloon and barometer to the summit of Everest in carefully controlled temperature conditions (the same as at ground level) and use the barometer to find the atmospheric air pressure p₂. Boyle’s Law says that, provided the temperature of the gas doesn’t change, the product of volume and pressure of the gas remains constant, so p₁v₁=p₂v₂ where v₂ is the volume of air in the balloon at the summit. So you can work out the volume v₂=p₁v₁/p₂. The volume will be greater than at ground level because the atmospheric pressure is lower at higher altitudes.