Stripping the screw usually only happens if a] you’re using a screwdriver that’s the wrong size, or b] you aren’t applying sufficient pressure. You may be able to remove the screw by using some fine/needlenose pliers, but I’d urge caution in doing so, and you might want to take this somewhere to have it done professionally if you’re at all uncomfortable doing so safely. Once you’ve loosened it, remove it by hand if possible.
Also, I’d recommend you replace the screw while you have the case apart. It is most likely a metric, m2 (as in, m1, m2, m3, etc. unrelated to the M.2 SSD form factor) x 3mm long, though the length may vary by about 0.5mm. You can sometimes find these at local hardware stores like Ace or a locally-owned non-Ace store. Larger hardware stores like Home Depot or Lowes (in the US) are less likely to carry metric screws that small.
Steps 4-5 appear to be unnecessary for this upgrade unless you’re disconnecting the battery for safety reasons. Is that why they’re present?