Can I upgrade my RAM from 2x8GB to 2x16GB?

Yes, but you have to replace the sticks, not add more. Pull out the old 8GB sticks and put in two 16GB sticks for 32GB total. Make sure your motherboard supports 32GB and check the QVL if you want to be safe, but any decent brand kit will work. Don’t mix different RAM kits — buy a matched 2x16GB kit to avoid stability issues.