Proper way(s) to substitute X for x in a CAS expression

[CR Haeger]

Sometimes, its useful to take an expression (say x^2-5) from the CAS and plot it in Function app.

I know that its possible to substitute X for x in a CAS expression using the following

f(x):= x^2-5 //which creates a CAS program f
Function.F1:=f //which saves X^2-5 to F1

Are there any other straightforward ways of doing this? The subst() command does not seem to do it.

Thanks