Soundfonts

To use the font, open the synthesizer. Select the sound font (press the Add button if it's not displayed). If it's not displayed in the box when you click Add then you need to put the sound font in the correct folder. See https://musescore.org/en/handbook/soundfonts-and-sfz-files#install to fix this.

Once you have the sound font in the list of the loaded sound fonts my advice depends on if the sound font is GM compatible. If it is, then move the sound font to the top of the list (if not already there) using the arrow button in the synthesizer window until it is at the top of the list. You can close the synthesizer at this point and all instruments should be using the new sound font.

If the sound font in not GM compatible, move it to second on the list. This will prevent random sounds from being assigned to the instruments. Close the synthesizer and open the mixer. Assign the sound from the sound font to each instrument. Note that the list of instruments in the mixer list all of the first sound font in the synthesizer's instruments, then all of the second sounds font's instruments and so forth.

Also note, that every score in the session will use the same sound fonts. If you add a sound font, it affects every open score. The next time you open MuseScore, the last session will be restored. You can see in the synthesizer that you have the option to save the currently loaded sound fonts as the default and load them at any time. To assign the sound fonts to a score you can click the Save to Score button, then Load from Score the next time the score is opened if you want. Keep in mind that every score uses the currently opened set of fonts. If you have different lists of fonts in different scores, only the last score that you press Load fonts will be shown and used.

All of this applies to sf2 and sf3 sound fonts. If you have SFZ sound fonts then read up on installing them and know that they are listed after all of the sf2 and sf3 sound fonts.

Use the mixer and select sound from one of those other soundfonts