If you are planning to put on new wallpaper on your walls then you must strip your old wallpaper as the new wallpaper sticks nicely to a clean wall than to a wall with old wallpaper on it. You can still put a new wallpaper onto an old one however it will peel off very quickly as the adhesive will not stay for long due the moisture present in it. Well, if you still plan to put wallpaper on an old one, you can re-paste the loose edges or tear it off. However you will still see lap marks if it is there. You can also smooth out the edges with sandpaper in order to apply the new wallpaper. Removing the wallpaper is not that difficult, there are ways by which you can try out.
Nowadays most of the wallpapers are strippable. Most of the wallpapers are made up of vinyl like fabric and have plastic like texture. Trying one of the corners of your wallpaper with a utility knife and pull it gently or with moderate care, it will easily come off. However if you have a non-strippable paper then you have to use a different method unlike pulling it out. You can try slitting and soaking or steaming in order to remove wallpaper.
You can make horizontal slits on the wall which has the wallpaper using the utility knife. Now using a sponge or a spray bottle, spray or put some soapy water on the wallpaper so that the adhesive soaks in the solution and loosen up. Using a wall scraper, you can scrape out or pull out the paper off the wall. When the wallpaper soaks up the solution or soapy liquid, it becomes easier to rip the paper off the wall as the adhesive loses its stickiness.
If you have dry walls, do not put any liquid solution on it or else you will damage the wall. Start by slitting horizontally as you did in the above method but instead of putting in the soapy solution; just scrape the paper off horizontally along the slits. If you are unable to peel the paper off using these methods, you can try out other methods which will help you remove wallpaper.
You must cut all power points first before starting the peel off job. Place cloths around the base of the wall; create small holes in the wallpaper using a wallpaper scorer so that the solution easily penetrates through the base of the adhesive. In a spray bottle add some fabric softener and hot water equally in proportion and spray it on the wallpaper. Leave it on for some time so that it gets soaked up and then start pulling out the paper upward from the bottom corner of the wallpaper. Repeat the process till you remove the entire wallpaper, then using a hot water and dish detergent clean off the remaining adhesive of the wall and finally rinse the wall with water and dry it using a towel. This is how you remove wallpaper.