Upgrade Ram. How exactly?

I have an Acer E5-751 gaming laptop! An I5, an Nvidia Geforce 840m and a 4gb ram memory are built in! Since I have now bought Gta 5 and realize that it is slightly jerky and the setting are at the lowest I wanted to upgrade my 4gb Ram memory to 8GB RAM memory! Is it better to buy a 4gb ram card or buy an 8gb memory card and sell the other one? Do I have to transfer any data? Looking forward to answers!

If you have enough slots, you can both clean, you can never have enough

And if it is the same RAM, you can use it in the dual channel is also faster

What is more useful?

That should not really matter, the difference is not that big. You do not have to transfer data, RAM is just a "temporary" memory, all important data is on the hard disk.