There is no way to change your stats in Runescape, people do say there is though but this is a commonly used scam to steal your Runescape account, by getting you to give them your username and password so they can supposedly use it on your account.
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Also another instance is downloading from a site, these will contain keyloggers and most anti-viruses don't pick them up, but you might get lucky, even if it is picked up the actual program is still useless, a keylogger ill record any key stroke on you keyboard, so if you log onto runescape the creator of it now has your username and password.