How do I access the leaderboard and change the deaths in a function?

You do not access the leaderboard, you create the leaderboard.

Leaderboards are made by creating an IntValue named "leaderstats" inside of the player, then inserting all other stats, such as NumberValues for Kills and Deaths inside of the leaderstats.

You can figure that out yourself. As to changing the Deaths when the Player dies, you can use the .Died event.

function humanoidDied(humanoid)
    local player = game.Players:GetPlayerFromCharacter(humanoid.Parent)
    if player ~= nil then
        player.Deaths.Value = player.Deaths.Value - 1
    end
end

for i,v in pairs(game.Players:GetChildren()) do --Fires the humanoidDied function for each player
    v.Character.Humanoid.Died:connect(humanoidDied)
end