Does upgrading RAM void my computer's warranty?

No, upgrading RAM won’t void your warranty as long as you don’t break anything. Most major prebuilt makers like Dell, HP, Lenovo, and ASUS allow user-replaceable parts, and “warranty void if removed” stickers on RAM slots are basically illegal in the US under the Magnuson-Moss Warranty Act. However, damage from snapping a DIMM slot or shorting a motherboard isn’t covered. Check boutique builders’ policies, but for mainstream brands, you’re fine.