I can't possibly believe that a PC would crash from a user upgrade. If that is the case, then the manufacturer would have a stranglehold over customer upgrades, which would - no doubt - be in breach of some trade law or other. We would certainly have heard something of that before now, and Dell would not be as popular as a result.
You can upgrade if you wish,provided you use compatible kit. EG you can't upgrade the CPU if you motherboard doesn't support it, you can add another HDD if you've a free bay etc. However, if you upgrade Dell, and lots of other suppliers, may refuse to honour warranties - K