If you have a wallpaper seam that is coming apart, don’t worry, you don’t have to replace the entire piece. Instead, you just want to reactivate the paste. Take a rag and dip it in warm water. Once soaked, you hold the rag on the area. Keep it there for about a minute or two, longer if you feel it’s necessary. Then carefully open up the gap. You need a larger gap or else you can’t apply the sealer.
Once you have a big enough gap, squeeze the seam sealer into the gap. If you want to, you can also use white glue. Then, once glued, press the wallpaper to the wall with a roller. If there is any excess cleaner, you can clean it up quickly.