...in your used language files (i.e. for german it's in /language/de-DE/de-DE.com_user.ini)
The text above that button (name, password, ...) is in templates language file i.e. language/en-GB/en-GB.tpl_gk_blackandwhite.ini
update: saw that k2 language file overwrite the *.com_user.ini file so you have to look in i.e. german /language/de-DE/de-DE.com_k2.ini