Hi,
It is not a problem to mix the two RAMs as it is a build feature rather than a performance feature.
Naturally for the RAM to run in dual channel mode it is best to have two sticks of the exact same modules of RAM. So you could consider a 2x8GB kit and maybe use the "old" RAM in a different machine.
Max RAM for your machine is 64GB of DDR4 2666MHz RAM (2x32GB)
You can insert faster RAM which will run at 2666MHz.
Example:
https://www.crucial.com/compatible-upgrade-for/hp---compaq/hp-15-dw1020nl#memory
Another upgrade you should consider is to upgrade the M2 SSD. At the moment you have a 128GB M2 SATA3 SSD in the notebook. You could upgrade to a much faster M2 PCIe NVMe SSD of your preferred brand and capacity. Something like the Samsung 970 Evo Plus would be consider the best but there are many great ones out there.
Hope it helps,
David
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