What is best reagent would you choose to convert ${\mathrm{MnO}}_4^{2-}$ to  ${\mathrm{MnO}}_4^{-}$

Oxidation state of Mn in ${\mathrm{MnO}}_4^{2-}$ is +6 and in ${\mathrm{MnO}}_4^{-}$ is +7. Mn gets oxidized from +6 to +7, therefore need a reagent that acts as a good reducing agent. Both chlorine water and ozone act as reducing agents and use any of them.

$2{\mathrm{K}}_2{\mathrm{MnO}}_4 + {\mathrm{Cl}}_2 \stackrel{∆}{\to }2\mathrm{KCl} + 2{\mathrm{KMnO}}_4$

$2{\mathrm{K}}_2{\mathrm{MnO}}_4 + {\mathrm{O}}_3 + {\mathrm{H}}_2\mathrm{O} \to 2{\mathrm{KMnO}}_4 + 2\mathrm{KOH} + {\mathrm{O}}_2$