Which class can be used to update user's data?
What user data? Do you mean user Profile? If so, use you can see some inspiration here.
If that's not it, maybe you have some of the latest versions of Epi, in that case you can use ApplicationUser by accessing UIUserManager.
In any case, add additional info about what exactly you want to update, which version of Episerver you are using and what kind of authentication you use.