[shirofuji]

get access to the ftp server of the website, edit the files inside the folder that contains, php and html files. Its not easy to do it on a real website. but if you have your own website you can try scanning it with nmap, brute-force the ftp login. and then once you get the login credentials you have access to the php and html files. this is much useful because you can also execute php codes that could harm the system.

you would need the ff to to what i said above:
nmap
thc-hydra
a good list of possible passwords written in a textfile
a lot of time
a lot of effort