You can upgrade, but match the soldered stick.

Yes. The free SO-DIMM slot is there for exactly this reason. But you’re not free to just grab any random RAM stick.

The soldered memory on your laptop is fixed at a certain speed (e.g., DDR5-4800) and timings. For the laptop to run both sticks in dual-channel mode—which is what you want for performance—the new stick needs to match that speed, latency, and voltage. If you install mismatched RAM, the system will either refuse to boot or fall back to single-channel (slower) and a lower common speed (likely the soldered one’s speed anyway, but sometimes the lower one). Check your laptop’s spec sheet or use CPU-Z to find the exact model of soldered RAM.

Also, some laptops cap total RAM capacity even if the slot physically accepts a larger stick. A 16GB soldered plus a 32GB SO-DIMM might only work as 32GB total, not 48GB. Look up the max supported RAM for your exact laptop model.

Bottom line: you can upgrade, but it’s not a “grab any stick” situation. Buy the exact same specs (speed, latency, type) as the soldered RAM, and you’ll be fine.