Clean the wallpaper with a sponge and a combination of detergent and warm water. Do not soak the wallpaper, but rather slightly dampen the sponge and use it to scrub off any dirt or grime.
Seal down any area where the wallpaper is peeling or lifting. Apply wallpaper glue to a paintbrush and brush the exposed backside of the wallpaper, then press it down firmly against the wall.
Prime the old wallpaper with a paint roller and primer. Apply the primer using long, even strokes.
Cover your bad wallpaper with new wallpaper. Use wallpaper glue and a paintbrush to apply the glue to the old wallpaper, then press on the new wallpaper. If you do not want new wallpaper, move on to the next step.
Paint over the wallpaper. For a smooth look, use the paint roller and oil-based paint. For a textured look, wait until the primer has fully dried, then apply paint by dabbing on with a sponge. Because patterned wallpaper may show up underneath the paint and look unsightly, test this technique on a small, hidden corner of the wallpaper prior to doing the entire wall.