It works both inside and outside of the radical symbol,
eq1 = 1/(2 A7 (1 + 
     ks) v0) ((-1 + A7) Imax - (1 + ks) v0 (A7 Km - 
       SuperStar[(Subscript[c, m])]) + Sqrt[
    4 A7 Km (1 + ks)^2 v0^2 SuperStar[(Subscript[c, 
       m])] + ((-1 + A7) Imax - 
       A7 Km (1 + ks) v0 + (1 + ks) v0 SuperStar[(Subscript[c, 
         m])])^2])
eq2 = (1 + ks) v0 SuperStar[(Subscript[c, m])]
eq3 = eq1/eq1[[5]]*eq2*
   Map[Divide[#, eq2] &, eq1[[5]]] /. {a_*Sqrt[b_] :> 
    Sqrt[Map[Divide[#, 1/a] &, b]]}