Change-of-Variable technique

Question

If the joint density for $q_{1}$, $q_{2}$ ... $q_{h}$ is $p$, what is the density for $y=q_{1}\psi_{1}+q_{2}\psi_{2}+...+q_{h}\psi_{h}$? where $\psi_{1}, \psi_{2}..\psi_{h}$ are given exogeneously