I'm assuming that the T5 acts similarly to the 60D. You have to go into the video menus and enable Manual Exposure. The 60D manual states that if aperture can not be set, press the Unlock button on the back of the camera. I would guess that the T5 has one. Time to dig into the T5 user manual.
In order to change the aperture setting the Movie exposure setting in the menu will need to be set to manual as BurnUnit mentioned.
Once in manual you will need to hold the Av[+/-] button on the back of the camera while rotating the main dial on top of the camera (the dial used to adjust the shutter speed).
Holding the Av[+/-] button will change the function of the main dial to adjusting the aperture until it is released. A snip from the T5 manual covering this is below.
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