Hello! I am having problems with regards to finding an integral.

The question is find the integral of sqrt(1-x^2)

i know that u substitution can be used and that sqrt(1-x^2)du=dx but after i substitute this I don't understand how it transitions from being integral of (1-x^2) to (1-sin^2u)du, like where does that come from. Any help would be greatly appreciated.