eqn=y''+p y'+q y

eqn /. y -> μ V

D[a[x] b[x], x]

y'

eqn // FullForm

Plus[Times[q[x],y[x]],Times[p[x],Derivative[1][y][x]],Derivative[2][y][x]]

(q[x] y[x]+p[x] y'[x]+y''[x] /. y -> μ V) // FullForm

D[y[x], x]