Allow other users to unlock record in approval process

Lock() and unlock() methods in the System.Approval Class let you lock and unlock records by passing in record IDs or sObjects.

Enable the feature by :-
Setup –> Process Automation Settings –> Enable record locking and unlocking in Apex.

Create a Detail Page Button and associate it with the Visualforce Page and place it on the Page Layout. 

Create a Visualforce Page

<apex:page standardController="Order" extensions="OrderLockUnlockController" action="{!unlock}">
</apex:page>

Create a Class

public class OrderLockUnlockController{
     private Id recId;

     public OrderLockUnlockController(ApexPages.StandardController stdCtrl) { 
         recId = stdCtrl.getId();
     }

     public PageReference unlock(){
         if (Approval.isLocked(recId)) {
             Approval.unlock(recId);
         }
                      
         PageReference pageRef = new PageReference('/'+recId);
         pageRef.setRedirect(true);
         return pageRef;
     }             
}