My dad bought this stupid HP desktop out of nowhere simply cause it was on sale and it had a 1 TB HD..... I yelled at him.

I'm curious If I can upgrade this and if I can how should I?

Specs:

OPERATING SYSTEM • Genuine Windows® 7 Home Premium 64-bit
PROCESSOR • AMD Athlon™ II X4 620 Quad-Core Processor
PROCESSOR SPEED • 2.60GHz
CHIPSET • NVIDIA MCP78V Chipset
CACHE • 2MB L2 Cache
BUS SPEED • 4000MHz System Bus
MEMORY • 8GB PC2-6400 DDR2 SDRAM memory
HARD DRIVE • 1 Terabyte 5400RPM Serial ATA hard drive
VIDEO GRAPHICS • NVIDIA GeForce 9100 Graphics with 256MB (It's integrated...oh how nice /sarcasm)