You cannot upgrade soldered RAM.

No. The soldered RAM is fixed at the factory. You cannot remove or replace it. That is the whole point of “soldered.”

What you can do is upgrade the SODIMM slot to match the soldered capacity (or exceed it), but if you want dual-channel to work, both sticks need to match in size and speed. So if your laptop has 8GB soldered, you can add another 8GB SODIMM to get dual-channel. If it has 4GB soldered, you can add 4GB or 8GB — but with 8GB you’ll run in single-channel for the extra 4GB (asymmetric dual-channel mode, which is better than nothing but not full dual-channel).

The only way to “upgrade” the soldered part is to buy a different laptop. So check what’s soldered before you buy. Future You will thank you.