Question

How to reduce h3po3 to h3po2

Answer 1

https://chemistry.stackexchange.com/questions/90914/why-is-hypophosphorous-acid-h3po2-a-strong-reducing-agent.

If you render the oxidation state of hypophosphorous as +5, then the electrons in the phosphorous-hydrogen bonds are attributed to the hydrogen atoms in those bonds. You can do that, but then the hydrogen atoms involved have oxidation state -1 not +1. So the reducing action of hypophosphorous acid is attributed to the negative oxidation state hydrogens.