How can I make my player's 'time alive' leaderstat reset when they die?

Make it sense if the health is 0, using if player.Character.Humanoid.Health == 0 then, like this:

game.Players.PlayerAdded:connect(function(player)
 local Stats = Instance.new("Model", player)
 Stats.Name = "leaderstats"
 local Timer = Instance.new("IntValue", Stats)
 Timer.Name = "Time Alive"
 Timer.Value = 0
 while true do
  wait(1)
  if player.Character.Humanoid.Health == 0 then
    Timer.Value = 0
    print("Player died")
  end
  Timer.Value = Timer.Value +1
 end

end)

Here's it:

game.Players.PlayerAdded:connect(function(player)
 local Stats = Instance.new("Model", player)
 Stats.Name = "leaderstats"
 local Timer = Instance.new("IntValue", Stats)
 Timer.Name = "Time Alive"
 Timer.Value = 0
local Rank = Instance.new("StringValue")
Rank.Parent = Stats
local ID = 1331531 --Switch this ID with your group's ID.
Rank.Value = player:GetRoleInGroup(ID)
Rank.Name = "Rank"
 while true do
  wait(1)
  Timer.Value = Timer.Value +1
 end

end)

Here's the script you can use to accomplish this:

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

    local Stats = Instance.new("Model", player)
    Stats.Name = "leaderstats"

    local Timer = Instance.new("IntValue", Stats)
    Timer.Name = "Time Alive"

    while true do
        wait(1)
        Timer.Value = Timer.Value +1
    end

    player.CharacterAdded:Connect(function(character)
        character.Humanoid.Died:Connect(function()
            Timer.Value = 0
        end)
    end)

end)

I added some indentation to help with readability so it's easier for you and others to understand. Also, I removed the line where you set "Timer's" value to 0 because it already defaults at 0. All that's happening is that it detects when the player's character loads in, then checks for when the character's Humanoid dies. When the Humanoid dies, it resets the value to 0.

Hope this helps.

so i dont know how to use that script it dont work