Like down sampling up sampling resizes the image but this time converts the image to a bigger size. This happens by adding new pixels to the image to increase its size. This process can happen in a couple different ways depending on what software you are using to upsample. Some softwares will simply borrow neighboring pixel colors and expand the image using these borrowed colors to insert new pixels where needed. Others will look at three or four surrounding colors and decided what color would be best suited for the new pixel color. While some softwares are better then others what ever method your software uses it is always guessing at what the image should look like and wont give an exact representation of the original image. This is why upsampling in general is a bad idea because it increase file size with out adding any quality, while that being said it is still important to understand how to upsample. Below is an example of a upsampled image.
samplingexplained 