In acidic medium, ${\text{H}}_2{\text{O}}_2$ changes ${\text{Cr}}_2{\text{O}}_7^{2-}$ to ${\text{CrO}}_5$ which has two $(–\text{O}–\text{O}–)$ bonds. The oxidation state of $\text{Cr}$ in ${\text{CrO}}_5$ is:

${\text{CrO}}_5$ (Chromium peroxide) has a "butterfly" structure. It contains one $\text{Cr=O}$ bond and two peroxide ($–\text{O}–\text{O}–$) linkages.Oxidation state of oxygen in $\text{Cr=O}$ is $-2$.Oxidation state of oxygen in peroxide bonds is $-1$.
$$x+1(-2)+4(-1)=0⟹x=+6$$
The oxidation state of Chromium is $+6$.