WARNING : Undertale has only 1 file to save you game so be carefull if you realy want to use this method then it will delete all of you progression i recommend to put the file0 in a secure folder on you computer then you will not lose your entire progression .

If you are too busy to play a game for 2 hours straight to do a special route to get a combat at the end i will show you the flowey time machine

you have to put the file0 in the %LOCALAPPDATA% of you computer and after you click on undertale you will see a file named file0 , delete it and put the new file0 you just have downloaded