If you mean, a soundfont, then download it to your Documents/MuseScore4/Soundfonts folder, then you can select in for any given instrument in the Mixer. For a VST, install it according to the instructions that came with the VST and select in in MuseScore the same way as for soundfonts. If you mean Muse Sounds, install from Muse Hub, and again, select in the Mixer in the same way (although they should be selected by default in most cases once you have installed them).
Comments
If you mean, a soundfont, then download it to your Documents/MuseScore4/Soundfonts folder, then you can select in for any given instrument in the Mixer. For a VST, install it according to the instructions that came with the VST and select in in MuseScore the same way as for soundfonts. If you mean Muse Sounds, install from Muse Hub, and again, select in the Mixer in the same way (although they should be selected by default in most cases once you have installed them).