Best tool & method for cutting steel tubes (table legs)?
A hack saw is a fine tool to cut fairly thin gauge tubing (which this seems to be). You may want to use a jig of some sort, such as a miterbox, to keep your cut square to the piece.

Tape around the diameter to minimize chipping of the finish.  File down the cut edge to remove the burrs from sawing, using a metal file, followed by emory paper.

If there is a finishing cap on the bottom, you may be able to pry it from the cut off section and reinsert it in the shorter leg.

There are alternatives for cutting tubing, such as a tubing cutter (but not especially well suited to steel - save for softer metals, such as copper), Dremel-type tool (small grinding system with "cutoff wheels", basically abrasive disks - slow, better for smaller items), abrasive grinding tools (somewhat crude and messy) and metal cutting miter saws (a bit of overkill unless you have it at hand).

For all of these cuttings (especially motorized ones) use safety glasses! It's all fun and games until someone gets a metal shaving in the eye.