Thank you both for your help!  I was able to get the trigger to work using the code Mukesh provided.  Vishal, I do have to do an after trigger because the UnitPrice field for Opportunity Line Items cannot be updated with a before trigger since Net Amount is a formula field and gets calculated after the record is inserted.  Also, my understanding is that I need to use "recalculate.Formulas" in order for Net Amount formula field to be set before it updates UnitPrice. I am struggling because I don't know how to determine the value of the Net_Amount__c field and check that it has updated the UnitPrice field.

Anyway, the trigger works in the sandbox and now I am attempting test code.  Can either of you review and see what I am doing wrong? It seems to be harder then the trigger itself!  This is what I have so far.  As you can likely tell, I have been looking at many examples to determine how to write the test case, but now have become confused.  Please point in the right direction.
 

@isTest 
public class oliUpdateTest 
{
     static testMethod void TestoliUpdate()
    {
        Test.startTest();
        List<Account> testAccounts = new List<Account>();
        Account testAccount0 = new Account(
            Name = 'Arbor'
        );
        testAccounts.add(testAccount0);
        Account testAccount1 = new Account(
            Name = 'Arbor Villa'
        );
        testAccounts.add(testAccount1);
        insert testAccounts;

   
        Opportunity testOpportunity = new Opportunity(
            Name                 = 'Arbor - Cypress Tree Opportunity',
            StageName            = 'Strategy',
            CloseDate            = Date.today(),
            AccountId            = testAccount0.Id
            
        );
          insert testOpportunity;

        Product2 testProduct2 = new Product2(
            Name                  = 'Cypress Tree',
            IsActive              = true,
            CanUseRevenueSchedule = true
        );
        insert testProduct2;

 

        Pricebook2 testPricebook2 = [SELECT Id, Name, IsStandard FROM Pricebook2 where  isStandard=true LIMIT 1];

        PricebookEntry testPricebookEntry = new PricebookEntry(
            IsActive     = true,
            Pricebook2Id = testPricebook2.Id,
            Product2Id   = testProduct2.Id,
            UnitPrice    = 0
        );
        insert testPricebookEntry;

        OpportunityLineItem testOpportunityLineItem = new OpportunityLineItem(
            OpportunityId          = testOpportunity.Id,
            PricebookEntryId       = testPricebookEntry.Id,
            UnitPrice              = 0,
            Quantity               = 1,
            Revenue_Years__c       = 3,
            Annual_Amount__c       = 5000,
            ServiceDate            = Date.newInstance(2020, 1, 1)
        );
        insert testOpportunityLineItem;
    }    
    System.assertEquals(0,testOpportunityLineItem.Net_Amount__c)
             
        Formula.recalculateFormulas(List<OpportunityLineItem> OpportunityLineItem);
      
       
       System.assertEquals(testOpportunityLineItem.UnitPrice)
 

}

These are the errors I am getting: