Since printer ink bonds to paper fibers, it is harder to remove compared to ink from pens. Do note that the pristine white of the paper will never be back. Before you begin, do check the manufacturer instructions. In case you cannot access those documents, try a succession of removal methods.
The first method is to take a cotton ball and dab the fresh ink. Most printers spray ink droplets. This will continue to be wet for a couple of minutes. It is possible to pick up the ink if you act swiftly after the paper gets printed. If this is done, the next step becomes considerably much easier. Whatever you do, you must not rub the paper in a vigorous manner. Doing this may rip the paper. Cheaper inks are easier to remove than their pricier counterparts.
The second step is to gently scrape paper with a razor blade or sandpaper. This is most successful if the ink remains on the paper surface. Scrape the paper's top level off by using ultra fine sandpaper or razor blade. Scraping should be done in only one direction. This is towards yourself. Chances of success are increased if the paper is thicker. If the ink is UV cured, it can be scraped off much easily. If the above method does not work, then the only option left is to use whiteout. Allow the compound to dry before using the paper again.
Yet another method is to use a cotton ball to apply acetone so that the ink gets smudged. This is especially applicable for laser printers. The paper will be smudged and gray, but new printing can be done on the substrate. The acetone should be kept distant from any heat source as it is a flammable compound. If the acetone contacts your eyes, skin, or mouth, quickly rinse with water. The latter should be lukewarm and do it for about 15 minutes. Do not pause during this procedure. Do not remove contact lenses if you wear them.
Rub acetone in with the tissue paper. This substantially increases the quantity of removed ink. Do note that about one-third of ink will remain as faint image. You should rub the tissue paper only once over affected area. Extra rubbing could tear paper. If you have an ultrasonic cleaning machine on hand, then place acetone-soaked paper inside it. More ink smudge will be removed.