Changing Image resolution is very easy now days; you can change the resolution or size of a picture by following ways,
The number of pixels
The file size
The number of pixels :- The Picture resolution, or their sharpness is determined by the number of pixels it has. More pixels will improve the resolution of a picture, and this will helps you to make a larger print without reducing visual quality and sharpness. But remember, the more pixels or resolution or sharpness of a picture will increase file size automatically.