If you put them into your GPS as waypoints/landmarks and add them to a route, your GPS will calculate the distance for you. Button pushing details are different for each GPS model, so that is left as an exercise for the reader.
If you have the points in UTM, and they are close enough together to ignore the curvature of the earth, you can work with them like XY Cartesian coordinates.
The old Pythagorean Theorem will let you calculate the distance like so
distance = sqrt( (X1-X2)**2 + (Y1-Y2)**2 )
Do a Google search on Pythagorean Theorem for prettier equations and pictures.