How to make OverheadGui change when players kill count reaches a certain number?

Hi so i have made a BillboardGui to go with players but i would like it to rank up a player and change what the Gui says after the player gets 15 kills and keeps going until the player maxes out at 300 how would i do this?

local billboardgui = game:GetService("ServerStorage"):WaitForChild("BillboardGui")

game.Players.PlayerAdded:Connect(function(player) player.CharacterAdded:Connect(function(character)

    local clonegui = billboardgui:Clone()
    clonegui.TextLabel.Text="Private"
    clonegui.TextLabel.TextColor3 = Color3.fromRGB(170,0,225)
    clonegui.Parent = game.Workspace:WaitForChild(player.Name).Head
    if player == 15 then --What is our kills value?
        local clonegui = billboardgui:clone()
        clonedgui.parent = game.workspace[player.name].head
        clonedgui.TextLabel.Text = "PrivateFirstClass"
        clonegui.TextLabel.TextColor3 = color3..fromRBG(231,206,12)

                end

end)

end)

Update: I have his kills value in a leader board script, if i were to add the kills value where the comment is will this all work out?