While the best advice is to not get wallpaper paste on finished wallpaper in the first place, it can happen if you don't carefully wipe down the wallpaper sheets with water when you are hanging them. Once the paste has dried, you have to carefully remove the paste; you can damage the wallpaper by rubbing on it or scraping it. Before removing any paste, test your method on a smaller, less noticeable area of wallpaper to prevent damaging larger pieces. Wallpaper manufacturers recommend a few methods that should lift the paste from your wallpapered surface.
Wipe the wallpaper paste gently with a damp cloth if the paste is not completely dried. If the wallpaper paste is dried, hold the damp cloth on the paste for a few seconds at a time to see if the paste will soften. Try dabbing at the spot and wiping away the paste if it becomes soft.
Scrape the paste gently with the edge of a paint scraper. Chip away small sections of the paste. When you get closer to the surface of the wallpaper, hold a damp cloth on the paste to soften it. Wipe the remaining paste away with the cloth.
Place a paper towel over the dried paste. Spray just a few squirts of window cleaner on the towel. Do not soak the towel; you don't want the window cleaner to run down the wall. Hold the towel on the spot for about 10 minutes to give it plenty of time to soak in. Dab the spot with a soft cloth after the paste softens.