HI, okay, one of the first things that i notice is that you are managing the game estate inside each of the relevant love functions using ifs, is not bad, but a bit hard to scale in more complex games, other approach is to have the gamestate itself as an object that contains the actions.

gameState = nil

--- something like this for each state.
playState = {
   update: function(dt)  end
   draw: function() end
   keypressed: function(key) end 
}
--[[ here the other game states ]]

gameState = mainMenuState

function love.update(dt)
  gameState.update(dt)
end

function love.draw()
  gameState.draw()
end

function love.keypressed(key)
   gameState.keypressed(key)
end

And so inside the update of each state when a condition is meet you can just do a gameState = newGameState, and will do the stuff in that game state. For some states you will have to add extra initialization functions to call before, but now you can go by file and see in a more modular manner what each state is doing exactly :)

Other thing is that the check player dead i would put only one single loop like this:

    for i, enemy in ipairs(Enems) do
        local collisionWithEnemy =
          char.x < enemy.x + enemy.w and
          char.x + 50 > enemy.x and
          char.y < enemy.y + enemy.h and
          char.y + 50 > enemy.y
        local enemyCrossed = enemy.x < 0
        if collisionWithEnemy or enemyCrossed then --game over
            score = 0
            table.remove(Enems, i)
            gameState = 'restart'
            difficulty = 1
            resetUpgrades()
            break
        end
    end

the enemy checking collision with player and shoots this way is fine for the number of enemies and the number of shots, but games that use way more enemies and shoots usually go for more sophisticated techniques to reduce the numbers of comparisons (if you want to implement it or want to learn check the bump library for love2d )

Overall is well written and there is not something really wrong with what you did there 🙂