Multivariable Calculus – Change of Bound Help!

I am having trouble following these steps in a reading on multivariable calculus.

Due to a change of variables: 

$ \displaystyle\int_t^T \int_t^s \theta_v dv \, ds = \int_t^T \int_s^T \theta_s dv \, ds$

Could anyone explain how you get the right equation? What is the ‘change of variable’ that is applied? Thanks!! Also $ t \leq T$ is given.