public class Api4 {
   public static void api(){
    Http h = new Http();
HttpRequest req = new HttpRequest();
req.setMethod('GET');
req.setEndpoint('https://demo1691926.mockable.io/Api12');
req.setHeader('Content-Type', 'application/json');
HttpResponse response = h.send(req);
system.debug('response  : '+response.getBody());
String json1= response.getBody();

       
DesearlizingJson desearilize1=(DesearlizingJson) Json.deserialize(json1,DesearlizingJson.Class);
System.debug('Phone is ' +desearilize1.phone);
System.debug('name is ' +desearilize1.Name);
System.debug('mobile is ' +desearilize1.MobilePhone);
      
 //trying to update deserialized json       
list<contact> lcon= new list<contact>();

List<contact> lcon1 = [select id,Name,MobilePhone,Phone from contact where LastName=:desearilize1.Name];
       for(contact c1:lcon1){
        
        c1.Phone=desearilize1.phone;
           lcon.add(c1);}
        update c1;
       
  }
}

Wrapper class

public class DesearlizingJson {
    public Decimal Phone;
    public String Name;
    public Decimal MobilePhone;

}

Thanks in Advance