Hello, I have some lowpoly game models with 256 and 512 textures. How can I make these models into highpoly CG models? How to make these textures into 2048 x 2048?
Welcome to BA 
Upscaling textures is much more difficult than down scaling. You can easily open your textures in Photoshop, GIMP, Krita, Pixelmator, Affinity Photo, ClipPaint, or even MS Paint and resize them to whatever size you want, but because you don’t have enough detail for the higher size, your result will be blurry
Basically: make them from scratch. Seroiusly: If you want something in better quality you have to buy or build it in that quality. A 6.55$ bottle of wine can’t be ever the quality as a 420$ bottle of wine…
(If you are curious: The numbers are from 256 * 256 / 10000 and 2048 x 2048 / 10000 
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And, “wine analogies notwithstanding,” you cannot create detail where none exists.
If you give a digital computer “many pixels” or “many polys,” and ask it to compute something with fewer pixels or polys, then there are plenty of algorithms that can do that. But it’s quite impossible to go the other way, because “there is simply nothing for a blind and ignorant digital computer to draw from.” There is no suitable “input.”
You have to be the one to interpret those models and creatively invent appropriate new ones. “Have fun!”
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There’s no real distinction between game models and cg models: Game models are CG models. Games have tighter poly budgets and texture limitations than models that don’t need to work in real time rendering scenarios, but are not distinguishable from any other 3D model for the most part.
Subdivision will increase the polycount and give you a higher resolution model to work from. The multiresolution modifier increases the polycount while allowing you to sculpt high poly details onto the subdivided polys (applying the subdivision modifier would have the same effect, but you can’t go down to a lower subdiv level after applying the modifier and I wouldn’t advise that approach if you plan to work with rigs on that model or implement it into a heavy scene - being able to lower viewport subdiv levels and raise render subdivision is a lifesaver).
For textures, as others have said, any image software will let you resize the texture to a higher resolution, but you’ll have to add the high res texture detail yourself. You may have some luck with AI based upscaling, but they can’t invent detail that’s been lost. And that’s a hefty size difference. You’re trying to increase the size of the 256 textures by 800%.