1-How can we change the invoices' serial numbers? 2-When we add time activity cost in the projects it's not reflected in the cost of the project, why and how to fix this?

It's pleasure to assist you today, aya-hesham-san3a.

Let me break down your questions a bit to make sure we're covering everything.

For your first question, serial numbers in QuickBooks Online (QBO) can be modified by going through each item of your Products and Services window. I'll show you how to do it below:

1. Go to Sales and choose Products and Services.
2. Select the item you want to change the serial number and click Edit.
   
3. From the SKU field, enter the correct serial number.
4. Click Save and close.
   

The items used on your invoices should now have the updated serial numbers. You might want to visit this article to learn more about adding product and service items to QuickBooks Online.

For your second question, the added time activity cost in the project should reflect on the Time cost by employee or vendor report once saved. In this case, we can try some basic troubleshooting steps to get this working.

Let's try using a private browser or Incognito window to isolate this issue. Since your web browser keeps files in its cache and saves the pages you have previously visited, private browser is a good place to start.
 

If it added time activity cost reflects on the report, please go back to your original browser and clear the cache. This is to make sure your browser functions more efficiently. Otherwise, you can try other supported browser to help narrow down the cause of this behavior.

You may find this article about tracking income, costs, and profitability by project useful for your reference.

Lastly, the decision whether to set all products in accounts or not depends on your business needs and how relevant it is to centralize your types of revenues by classes. You can consult your accountant for more expert advice.

Also, you can learn more about using class tracking in QuickBooks Online by visiting this article.

Feel free to comment below if you have further questions. I would appreciate the opportunity to help you with your concerns.