Update form in ionic4

Appflow
1

my html page

<form [formGroup]="registerForm">
    <ion-card *ngFor="let user of (test)?.results">
         <ion-item>
            <ion-label floating>Name</ion-label>
            <ion-input formControlName="name" type="text" value="{{ user.name }}"></ion-input>
          </ion-item>
            <ion-item>
            <ion-label floating>Username</ion-label>
            <ion-input formControlName="username" type="text"  value="{{ user.username }}"></ion-input>
          </ion-item>        
          <ion-item>
            <ion-label floating>Password</ion-label>
            <ion-input formControlName="password" type="text" value="{{ user.password }}"></ion-input>
          </ion-item>
        
    </ion-card>
      <ion-button expand="full" (click)="update()">Update</ion-button>
    </form>

ts file

update(){

this.registerForm.patchValue({
 name_new:this.registerForm.value.name,
 username_new:this.registerForm.value.username,
 password_new:this.registerForm.value.password

});

let id = this.activatedRoute.snapshot.paramMap.get('id'); 
var link = 'http://localhost/curdForm/index.php/Login/update';
var myData = JSON.stringify({name:this.name_new,username: 
this.username_new,password:this.password_new,id:id});    
this.http.post(link, myData)
.subscribe(data => {
this.data.response = data["_body"]; 
console.log("success!");
if(this.data.response==null){
 alert("Not Updated");
}
else{

 this.navCtrl.navigateForward('admin');
}

}, error => {
console.log("Oooops!");
});

}

I need to update the form values. If the form values not changed and click the button, then text values become null values. I need the original value of text field from the form, if it’s not changed.