I'd love to help you in selecting expense transactions in QuickBooks Self-Employed. However, I'd like to ask what type and where do you want to select the expense. Any further details on this matter are much appreciated.
If you're referring to selecting a specific date for expenses on the Transactions page, you can do that by customising the date and the type of spending.
Here's how you can do that:
Click on Transactions from the left pane.
Choose Spending as the transaction type.
Select the expense that you want to show.
Hit on Custom so you can choose your desired date of transactions.