Hello 

Using Code: 

if you have your code written directly in the trigger or in a class designated without sharing, then the apex code can update records that are locked, regardless of the user who caused the trigger to fire. I tested by creating a trigger on Contact that updates a field on the Contact's Account. When the Account is locked, I can edit the contact record with a non admin user and the field on the Account gets updated.
If you use code in a class marked with sharing that is called by the trigger, then you will get an exception. If the trigger was executed by an Administrator, then the code will always run

For Example I have added to code unlock Opp
 

public class UnlockRecordDuringApprovalCon {
    Opportunity objOpportunity;
    // String objOpp;
    String currentuserId = UserInfo.getUserId();
    
    public UnlockRecordDuringApprovalCon(ApexPages.StandardController controller) {
        objOpportunity = (Opportunity)controller.getRecord();
    }
    
    public void processRecord() {
        Approval.UnlockResult unlockedRersult = Approval.unlock(objOpportunity);
        // Iterate through each returned result
        if (unlockedRersult.isSuccess()) {
            // Operation was successful, so get the ID of the record that was processed
            System.debug('Successfully unlocked opportunity with ID: ' + unlockedRersult.getId());
        }
        else {
            // Operation failed, so get all errors                
            for(Database.Error err : unlockedRersult.getErrors()) {
                System.debug('The following error has occurred.');                    
                System.debug(err.getStatusCode() + ': ' + err.getMessage());
                System.debug('opportunity fields that affected this error: ' + err.getFields());
            }
        }
        System.debug('currentuserId ' + currentuserId);
        
    }   
}

Admin View: 
Setup->Create-> Workflows and Approvals -> Process Automation Settings

and set checkbox Enable record locking and unlocking in Apex as true

Mark Best ANSWER if its Solved.

Thanks
karthik