Some Japanese hand saws can be sharpened. If the teeth of your saw are not impluse-hardened (blue tipped) than you should be able to sharpen them using a feather file.
Feather files come in several sizes for different tooth counts. The three most common are a no. 3 (75mm) for dozuki saws (reinforced spine, fine teeth for crosscutting), and a no. 4 (100mm) for ryoba saws (dual-edge blade for crosscutting and ripping) and no. 5 (125 mm) for the more aggressive rip and cross cut saws.
To sharpen with a feather file, clamp your saw in a vise with the teeth just above the jaws. Because these teeth are sharpened in an alternating left-right pattern, you'll sharpen every other tooth from one side, then flip the saw around and sharpen the remaining teeth. Push the file across each individual tooth 3 or 4 times, more if necessary to repair damaged teeth. After completing this step, file tiny secondary bevels on the tips to avoid having them break off during use.
