Should I replace the HDD with an SSD first, or upgrade the RAM?

Replace the HDD with an SSD first. It’s the single biggest speed upgrade for an old laptop—your computer is slow because the hard drive is the bottleneck, not because it’s low on memory. An SSD makes everything snappier immediately, while doubling RAM gives a smaller boost for more money. Only upgrade RAM first if you have soldered RAM and know you need more memory for specific work.