I own a DELL XPS 15 9550 for two years now.
I would like to upgrade my SSD from 256GB to 1TB SSD, what is the process?
and secondly, battery drains pretty quickly, is there any better battery I can buy?
Any M.2 2280 drive will work - NVMe or SATA. You'll want to image your current system to an external hard drive (Macrium Reflect is a good tool to use), and then boot the system from the Reflect recovery media you create, and restore the image. NVMe is faster; SATA is cheaper (you'll likely pay upward of $600 for an NVMe 1T drive; SATA will be half that).
There are two battery sizes - 56 WHr or 97 WHr. If you have a 2.5" drive in the system, you're limited to the former; if you have M.2, you can use the larger capacity battery (your setup screen will show which you have). At two years old, you're likely seeing a battery near the end of its lifecycle -- particularly if the runtime is gradually decreasing.
Thank you for the detailed answer!
I think I will order the NVMe 1TB by Samsung, as for the battery: I am not sure how to check what battery I can pruchase and also where can I buy?