All the value fields are editable and the remaining sheet is password protected to disallow any unintended edits. Still, if you want to do a knowledgeable edit, the password is ‘euroscoreii’.
The calculator is based on this paper: http://ejcts.oxfordjournals.org/content/41/4/734.full (freely available).
PS: EuroSCORE is basically a mathematical model based on certain parameters to predict a probabilistic mortality rate of a heart patient. It helps in surgical decisions.