I'm here to help you to match the check numbers on your Expenses/Vendor and Banking pages.
If you're seeing check numbers in your downloaded bank transactions, I recommend not changing/editing them since it's provided by your bank.
But if the given field is empty, you can enter the check number in it. Let me show you how.
1. On the left panel, click Banking.
2. Select the account where the check located.
3. Find and click the check.
4. In the Check Number field, enter the check number that matches to your expenses.
5. Click Add.
6. Click Yes when prompted to record the changes.
You can see the screenshots below to see exactly what I mean.