How to change a player's appearance via gui button?

hello!! im fairly inexperienced at scripting, but im making a game with a custom StarterCharacter. On his mouth, there are several decals for different facial expressions (a closed mouth, a frowning mouth, etc.)

I tried to make a gui button that would allow you to change his facial expressions at will, but it doesn't work. After looking it up I'm realizing you can't change his face via gui buttons unless its a local script, but thats the problem- I want everyone in the server to be able to see it when they change their own facial expression, not just local.

I was thinking maybe I have to do something with a remote event/FireServer function? but I've only ever seen people do that for ingame currency (ex. when someone buys something ingame and it takes points away from your player)

Again, i'm a little new at scripting so I probably sound a little clueless. I'm not asking anyone to make a script for me, but I'm wondering what direction I should take with this, because I couldn't find anyone with a similar problem when I googled it.

If it helps, I can show the script im currently using (though i already know why its not working, im just unsure of how to remake it.)

thank you!!