Upgrade means putting more than 2-3 times than the stock, at in 5k, i recommend to buy first a good separates and power them with your head unit, together with the stock rear speaker, it will give some change but so if you have another budget, get some amp to help your head unit driving them, a 4 channel amp is a good choice, seps are range from 3600 to 4800, basic components, and these is the best selling price in our shop.
But if your plan is just want to upgrade it at once, never to amplify it later, so just buy a 2 sets of full range speakers, 2-3 way is the best, and it range from 2300-2500, just think what you want sir, visit our shop and hear this things if you have time and give you more recommendation.