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]]}