Locate matching wallpaper. Hopefully, you saved any extra from when you initially put up the wallpaper. Otherwise, you'll have to return to the store where you bought it.
Look at your spare wallpaper and your torn section side-by-side so that you can determine what portion you need to replace. When you see the area you need on your fresh roll, tear a larger section from it. Tearing the paper allows for thinner edges that will more seamlessly blend in with the existing paper.
Cut away any loose flaps of wallpaper surrounding the tear on the wall by using a razor blade. Leaving a flap in place and gluing the patch over it may cause the repair patch to bubble up later.
Mix the wallpaper paste using the instructions on the container. Use a small paintbrush to apply a thin coat of paste to the back of the torn-off patch.
Carefully stick the patch over the torn section and smooth it out with your hands. If it needs to be slightly adjusted into place, do it immediately while the paste is still wet. Use a slightly damp paper towel to wipe away any excess paste and to smooth out the perimeter edges of the patch.
Allow the patch to completely dry before touching it again.