How do I update my quickbooks online with the info from my desktop quickbooks 2020 program?

What do you mean with "update"? Do you already have data in your QBO account? or do you need to convert data from QB Desktop to a new blank QBO account?

At this point, my quickbooks 2020 desktop program has all my estimates up to September 6th 2023 but the latest date on my quickbooks online is May 13th 2023. I thought that the 2 programs would "sync up" but I guess I was wrong. Now I'd like to manually sync them together but I don't know how, thank you for your reply.  

Welcome to the Community, @nickcappadora-gm.

I'll help you migrate your data from QuickBooks Desktop (QBDT) to QuickBooks Online (QBO). Before doing so, make sure your QBO account is 60 days old or less.

Here's how:

1. Sign in as a primary admin to the company file you need to import.
2. Go to Company, then select Export Company File to QuickBooks Online.
3. Choose Start your export.
4. Sign in as an admin to your QuickBooks Online company.
5. Click Choose online company, and select the QuickBooks Online company you want to replace with your company file.
6. Select Continue.
7. If you track inventory in QuickBooks Desktop:

• Select Yes to bring them over, then enter the as of date.
  • Note: Use the first day after your last tax filing period as your inventory start date.
  • Select No if you want to set up new items later in QuickBooks Online.

1. Enter Agree in the text field, then select Replace.
2. Click Yes, go ahead and replace the data, then select Replace.

For future reference, you can read this article about fixing errors when migrating to QBO if you get one: Fixing errors when you move from QuickBooks Desktop to QuickBooks Online.

Feel free to comment below if you have questions about migrating data from QBDT to QBO. I'm always here to help. Have a great day.

@nickcappadora-gm 

Why did you still use your QB Desktop after converting data to QB Online? QB Online and QB Desktop are 2 different products. If you only need to use QB Desktop for a single user, you can use the trial version for QB Desktop 2021 Pro if required. Otherwise, you need to migrate data manually and regularly from QB Desktop to QB Online with a 3rd party tool.