Take apart your Uni-ball Impact RT pen by gripping the shaft of the pen and applying counter-clockwise pressure to the front part of the pen. This action should unscrew the front and allow you to take it off.
Grasp the old refill, which should now be sticking out of the front of the pen. It should easily slide out. Place it in a resealable plastic bag to ensure that the ink doesn't leak onto anything.
Reassemble your pen by screwing the top back on to make certain no pieces are lost while you're searching for a new pen refill.
Visit an office supply store to purchase a new refill, making sure to bring your old refill with you. Search for the section containing Uni-ball pen refills. Once you find them, look for the refills for the Impact RT. Compare the new refill in the package with the one in your bag to be sure they look the same.
Disassemble your pen by unscrewing the top again once you have purchased the new refill and have it with you. Slide the new refill in, back first, so that the tip is in its proper place at the front of the pen.
Screw the front of the pen back on over the new refill. Write on a scrap piece of paper to get the ink flowing; the pen is now ready to use.