Question What change should I make first ?

Why do you want to make a change?

Reason: problems, performance, future growth?

Update your post to include full system hardware specs and OS information.

PSU: make, model, wattage, age, condition (original to build, new, used, refurbished)?

Disk drive(s): make, model, capacity, how full?

Attached peripherals?
 
My config (6 years old, only used to play games):

Motherboard: Asus B450M Gaming
CPU: AMD Ryzen 5 1600 3.2Ghz
GPU: GTX 1660 6Gb
PSU: Corsair ATX 600w
RAM: 2 x 8GB 2666Mhz DDR4

I want to change because the fps in games is very low at the moment.