The Web UI is represented by one file : index.hml.gz
The full package with all languages is available here
But it may not fit your available size on flash, so you can select a single language package here which is lighter and will fit smallest boards.
Upload index.html.gz
Using embedded uploader
If the embedded is not visible, you can access to it using:
http://your_IP_address?forcefallback=yes
anytime
Using Web UI
You can upload file using Web UI
Finalize update
Refresh the page to http://your_IP
to display updated Web interface.
Note: you can also upload a customized favicon.ico