Change of variables and fiber bundles

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The random variable X has some density p(x) in Lebesgue measure and I know that X=f(A, B), for example X=A^2+B^2. How can I find the corresponding density over A, B if I assume it is distributed uniformly over each fiber bundle? For example for X=A^2+B^2 the density would be uniform over each circle around the origin but what exactly would be the density? Ideally I would like to find a general approach for any f (that fulfills some regularity conditions).