It depends on what you're updating and how much data you have. Here are a few examples.
A) If it's just CPU, motherboard and RAM then you don't need to do anything except reinstall Windows (hopefully you have your vauable data NOT on the windows partition).
B) If you are buying a new PC then setup the new how you want it. Then transfer all the data over the network to the new PC
C) If you are buying new HDDs then you have two options.
a. Leave the old storage connected and connected the new storage- Then transfer the data from old to new (if you have enough sata ports)
b. If you don't have enough sata ports you would need to transfer the data to a temporary storage location, then copy it back again.