I assume you mean hardware upgrade.
There are 4 main things you can upgrade:
Hard Disk, Memory, CPU and Graphics card.
The hard disk is straightforward, it is fairly easy to fit a bigger hard disk, or a second hard disk.
The memory is fairly easy, the most difficult thing is to identify the TYPE of memory for your PC.
Memory is changing all the time, such as the number of pins, voltage, speed etc so it is vital to get the right memory for your motherboard. Once you have more memory fitting it is easy.
CPU is more difficult. If you have an old PC (even a year old) you cannot just go out and by a new CPU and fit it. Like memory, CPUs are changing all the time with number of pins, speed etc.
Most motherboards support a RANGE of CPUs (between a certain CPU speed) but as CPU technology moves on so quickly it may be dificult to buy a CPU that fits your motherboard (they have stopped making them).
It may also be that your motherboard is running the fastest CPU it can so you have no upgrade potential.
Often if you want to fit a faster CPU you have to buy a new motherboard. If you buy a new motherboard it probably needs different memory so you finish up buying a motherboard, CPU AND memory.
Some people just go out and buy a new PC.
Fitting a graphics card is also easy, the difficult thing is to decide what one to fit as they range in price from �25 for a basic one up to �400 for a fast gaming graphics card.
You need to decide what you use the PC for to help you decide if you want to upgrade the graphics card.
To be honest it is often easier to go out and buy a new PC (or at least the system unit) than to get involved in upgrades.