I don't quite know what you are asking but, if you are thinking about replacing your current hard drive with another one then the process is simple. Open the tower or computer case make a note of what and how the cables are connected to the current hard drive, then take it out and replace it with the new hard drive using the same connection scheme that the old one used.
Now if you are thinking about getting rid of the computer all together, then it is a good idea to carefully remove the hard drive without touching the circuit board on the hard drive. From there you have various options either putting the removed hard drive in another computer or purchasing an enclosure if you want to use the removed hard drive as an external hard drive. But once again you have various options to choose from.