Uploading a soundfont into Musescore

Open Edit->Preferences and in the General tab you will see a series of labels in the bottom left corner. One of these is Soundfonts. Look at that directory and put the soundfont into that directory. You can then open the synthesizer, click the add button, select the soundfont, add it and move it down so it isn't first in the list. Otherwise it will mess up all existing scores.