Dear all,
Despite many a Google search I am unable to find the way for the following type of substitution. In a differential equation for the function u the independent variable is x. The variable x is to replaced by y^2. When I do it by hand, then from y=x^2 the first order derivative is performed like du/dx=du/dy*2x because later the x comes to be be multiplied by x from the diff.eqn. so that it is replaced by y.

In Mathematica I assume that the substitution has to be y=Sqrt[x]. Now if the expression eqn describes the diff.eqn. as a sum of terms like D[u[x],x] , second order derivative and u[x], I am unable to find the way to perform the substitution from x --> y in eqn

Could anyone hint me as how this is done?

Best regards,
ChG