Use Batch Apex

Use Batch Apex from (module - Asynchronous Apex)

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SOURCE CODE1: LeadProcessor

public class LeadProcessor implements Database.Batchable<sObject> {
    public Database.QueryLocator start(Database.BatchableContext bc) {
          return Database.getQueryLocator([Select LeadSource From Lead ]);
    }
    public void execute(Database.BatchableContext bc, List<Lead> leads){
            for (Lead Lead : leads) {
                lead.LeadSource = 'Dreamforce';
            }
        update leads;
    }
    public void finish(Database.BatchableContext bc){
      } }

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SOURCE CODE2: LeadProcessorTest

@isTest
public class LeadProcessorTest {
     @testSetup
    static void setup() {
        List<Lead> leads = new List<Lead>();
        for(Integer counter=0 ;counter <200;counter++){
            Lead lead = new Lead();
            lead.FirstName ='FirstName';
            lead.LastName ='LastName'+counter;
            lead.Company ='demo'+counter;
            leads.add(lead);
        }
        insert leads;
    }
    @isTest static void test() {
        Test.startTest();
        LeadProcessor leadProcessor = new LeadProcessor();
        Id batchId = Database.executeBatch(leadProcessor);
        Test.stopTest();
    } }