The easiest way is to just throw it into Preview or an image editing program and increase the size.
But there’s a problem with that.
If you had 32×32 sprites and doubled the image (sprite) sheet you’d end up with 64×64 sprites — but you’d also end up with sprites that aren’t crisp and clear. Going down in size is usually okay, going up in size introduces blur. The more you increase the size, the worse your image looks.
If you created the sprite sheets from individual sprite images, the largest size you can use for a sprite (without degrading the look) is however large your source image is. Which is why I now ask whoever’s doing my artwork to give me sprites that are at least 256×256.
If you want your sprite to be 32×32 on an older iPhone, that means on a newer iPhone you’ll need a 64×64 sprite, and on a retina iPad you’ll need 128×128 — all will look the same relative size on the devices, but when you have more pixels on the screen, your artwork needs to increase as well.
By getting artwork at the maximum size you’ll need, you can easily downsize the artwork to the other sizes without ending up with fuzzy artwork.