How can I change an expense transaction from a positive number to a negative number?

Hey there, @alex-panteli-apx.

Is this transaction downloaded from your bank or manually imported in QuickBooks? 

If it comes from your bank, you'll have to exclude it. Then, enter it manually in the program. Here's how to exclude: 

1. Go to the Transactions menu.
2. Find the transaction you want to exclude and select it to expand the view.
3. Click the Exclude this transaction checkbox.
4. Select Save.

Once done, follow these steps to manually add them in QuickBooks Self-Employed (QBSE): How to enter sales income and expenses in QuickBooks.

If you imported the transaction through a CSV file, you have to exclude it. Then, import it again to QuickBooks. Just make sure that the format for its amount is negative. This way, it will be classified as an expense. 

When you're done, review and categorize your transactions to put them in your accounts.

I'm always here to help if you have any other concerns or questions. Just tag my name in the comment section and I'll get back to you as soon as I can. Have a great weekend.