Hi,
Please try the following code；
function formOnLoad(executionContext){
var UserId = Xrm.Page.context.getUserId();
var data = {
"firstname":"Steve",
"lastname":"Zhao",
"fullname":"Steve Zhao"
};
Xrm.WebApi.updateRecord("systemuser", UserId, data).then(
function success(result){
console.log("Sucess");
},
function (error) {
console.log(error.message);
}
);
}
