In the conversion of ${\mathrm{K}}_2{\mathrm{Cr}}_2{\mathrm{O}}_7 \mathrm{to} {\mathrm{K}}_2{\mathrm{Cr}}_2{\mathrm{O}}_4$ the oxidation number of the following changes

${\mathrm{K}}_2{\mathrm{Cr}}_2{\mathrm{O}}_7^{-2}⟶{\mathrm{K}}_2{\mathrm{CrO}}_4$

In the reaction, Oxidation states of Potassium, Chromium and Oxygen on both the sides respectively are +1, +6 and -2. There is no change in the oxidation state of any element.