Hi:
If you enter the serial number of the notebook in the box at the link below, it should tell you how much memory was installed.
HP PartSurfer
Since we do not work for or represent HP, we are not authorized to discuss warranty terms.
Generally speaking, you will not void the warranty unless you damage something in the process of upgrading parts.
There is a service manual and a service video gallery to help you open up the notebook as safely as possible.
Maintenance and Service Guide HP 250 G7 Notebook PC
HP Support - YouTube