U-Substitution

After determining what u will be, how do you find du? Is it the derivative or antiderivative of u?
Also, do you replace u with the function it represents in only indefinite integrals, and not in definite?

Well, $\frac{du}{dx} is a notation for the derivative so I guess that du by itself is the derivative times dx. For example, if u=x^2, then

Also, do you replace u with the function it represents in only indefinite integrals, and not in definite?

I guess you’re referring to the translation the expression of u that is produced after integration. If so, then you don’t replace that in a definite integral - you just use your new bounds to compute the numeric value.

If I sound a bit equivocal in these responses, it’s because it’s much easier to answer questions in the context of a specific example.