Dynamic texture resize

User "equalsequals" said on another post: "You should really use 2 resolutions, one for older gen devices and one for retina. The standard way of doing things is to have a low and high res version of the sprites. This is because on the older devices the retina-optimized graphics may look great but they will be double the size you need them. This is not only bad for memory, but processing as well. If you use run-time arguments to determine the device generation, you can then dynamically load textures and resize sprite geometry as needed."

My question is: how do you "dynamically load textures and resize sprite geometry as needed." on run-time.

Many thanks to anyone who answers.