If the wallpaper is only torn…
We can consider that the wallpaper is torn when a piece is only hanging, but that it can be folded down on the wall to fill the hole. In this case, it suffices to coat the torn part with adhesive for tapestry connection. Do not put too much at the risk of completely moistening the piece of wallpaper and tearing it. Also remember to put glue on the wall. Then, simply place the part to be glued gently, paying attention to any patterns. With a brush or a clean cloth, gently press on the edges of the piece returned to its place then pass with a joint roller which will make the connections invisible. Finally, with a slightly damp sponge, you can remove any excess glue that protrudes from the edges.
… Or completely torn off
If the piece of tapestry is completely removed from the wall, it is necessary to completely restore it. For this, the principle is to provide a square of wallpaper larger than the hole to be filled. After having taken care of the connection with the reasons, in case the tapestry is not united, use a cutter to cut a square on the two levels of the paper: the new and the old. Then, it remains to paste the new portion and to apply it on the now bare wall. With a joint roller, you can smooth the edges and make the repair completely invisible, before passing a damp cloth or sponge to remove any excess glue that may appear.