#Step 1 : Create a apex class ‘dynamicFieldUpdateEg‘ and paste the following code.

public with sharing class dynamicFieldUpdateEg {
    /**
    * Webkul Software.
    *
    * @category Webkul
    * @author Webkul
    * @copyright Copyright (c) 2010-2018 Webkul Software Private Limited (https://webkul.com)
    * @license https://store.webkul.com/license.html
    */
    public String selectedField {get; set;}
    public String fieldValue {get; set;}
    public String accName {get; set;}

    // Get all fields if string type and which is updateable dynamically by using schema class.
    public List getAllAccField() {
        List  options = new List();
        Map <String, Schema.SObjectField> fieldMap = 
 Schema.getGlobalDescribe().get('Account').getDescribe().fields.getMap();
        Map<String, String> preDefinedMap = new Map<String, String>();
        for(Schema.SObjectField sfield : fieldMap.Values()){
            String strType = string.valueOf(fieldMap.get(sfield.getdescribe().getname()).getDescribe().getType());
            if(strType == 'STRING' && sfield.getdescribe().isAccessible() ==  true && 
               sfield.getdescribe().isUpdateable() == true) {
                   options.add(new SelectOption(sfield.getdescribe().getname(), sfield.getdescribe().getlabel()));
               }
        }
        return options;
    }
    
    public void saveData() {
        try {
            Account acc = new Account();
            acc.Name = accName;
            acc.put(selectedField, fieldValue); 
            insert acc ;
        } catch (Exception ex) {
            system.debug(ex.getMessage());
        }
    }
}

In above example we put selected field with entered value from visualforce page by using following code.

acc.put(selectedField, fieldValue);

#Step2 : Now create a visualforce page as follows :

<apex:page controller="dynamicFieldUpdateEg">
   <!-- 
       /**
         * Webkul Software.
         *
         * @category Webkul
         * @author Webkul
         * @copyright Copyright (c) 2010-2018 Webkul Software Private Limited (https://webkul.com)
         * @license https://store.webkul.com/license.html
         */
     -->
    <apex:form>
    <table>
     <tr>
      <th> Enter Account Name</th>
      <th> Select Field </th>
      <th> Enter Value </th>
      <th> Action </th>
     </tr>
    <tr>
     <td>
      <apex:inputText value="{!accName}"></apex:inputText>
     </td>
     <td> 
      <apex:selectList size="1" value="{!selectedField}">
        <apex:selectOptions value="{!AllAccField}"></apex:selectOptions> 
      </apex:selectList>
     </td>
     <td>
      <apex:inputText value="{!fieldValue}"></apex:inputText>
     </td>
     <td>
      <apex:commandButton action = "{!saveData}" value="Save"/>
     </td>
   </tr>
  </table>
 </apex:form>
</apex:page>

That’s all about dynamic field mapping in salesforce apex class, still if you have any further query, feel free to contact us, we will be happy to help you
https://wedgecommerce.com/contact-us/