Yes. If you can connect to the Maslow via WiFi and get to the WebGUI, you can delete and upload files from there. The Maslow.yaml and index.html.gz can not be called anything else. Depending on the firmware you have installed, you can rename files or delete them before uploading new files.
You can also set it in boot-mode to re-install firmware via USB.
More info on that here: Trying to Install FluidNC v1.0 - #22 by bar