Opening the panel cover is simple.  One can try to figure it out but it's always better to refer to the User manual (already linked by Brad) Either way of upgrade, you will have to open the panel and work inside.  I can't speak for your knowledge but I feel adding an NVMe card on to an adapter outside, then open up the computer to install it onto the PCIe slot will be simpler than try to clone the current drive with some USB adapter, then open up the machine, replace the current drive with a larger capacity drive. While I could do both, I understand the fear, or hesitation of a new user.  I suggest to read the manual first, then decide for yourself if you can perform installation and removal of components. Post back if you have question and needing suggestion to obtain the proper adapter for DIY.
I have a 1TB SSD in my Optiplex 3020. Is it possible to add another one?
Adding a second SSD to Optiplex 3020