Or how can I reset without knowing the current one (user forgot password)?
If you want to change a password using the UserManager but you do not want to supply the user’s current password, you can generate a password reset token and then use it immediately instead.
string resetToken = await UserManager.GeneratePasswordResetTokenAsync(model.Id);
IdentityResult passwordChangeResult = await UserManager.ResetPasswordAsync(model.Id, resetToken, model.NewPassword);