Your title should be specific! Describe your problem concisely.?

From what I see, there's a little couple of errors.

When you fire RemoteEvents to the server, just make sure you capitalize it right, so instead of fireServer(), it should be FIreServer()

As well as that, to change the teams, you need to do that on the server, so it shows across everyone, so you'd need to send the server some info too. (If you change the player's team on the client, it only shows their team change to themselves, no everyone else)

Local Script

local Player = game.Players.LocalPlayer
local Teams = game:GetService("Teams")

script.Parent.MouseButton1Click:Connect(function()
    if Player then --Just to note, this isn't needed as it's on the client. You can still keep it in though.
        game.ReplicatedStorage.TeamChange:FireServer(Player) --Capitalize that "F"! Also, send the player to the server so they can change the team!
    end
end)

Server Script

game.ReplicatedStorage.TeamChange.OnServerEvent:Connect(function(Player)
    Player.Team = game.Teams.Hostiles
    Player:LoadCharacter()
end)

All in all, you just need to remember how the "grammar" involved with RemoteEvents is, and that teams need to be changed on the server. Hopefully this helps you out, and if there's some issues with it, lemme know!

EDIT: I am aware that he has used this answer for his question. He has told me he just didn't know how you accepted answers for the question.