Welcome to the Community. I'm here to provide insight into changing the word pledge.
Generally, pledges will appear in QuickBooks Online when a user has selected that their organization is a non-profit. If you're needing to change the name of the transaction that's sent to your customer/member, you can customize a template to do this. Here's how:
Select the Gear icon and click Custom Form Styles.
If you wish to modify an existing style, select Edit in the Action column. Should you be wishing to create a new template, click New style in the top right-hand corner and choose INVOICE.
Choose Content in the upper left of the screen.
Click the top box of the preview panel to the left.
Edit the form name to your desired term, such as invoice.
Select Done.
Now, you'll just need to select that template when printing or emailing the transaction and your emailed and the name will change. If this is the default template, your transaction will have the desired name automatically.
I've included a brief video to serve as a visual guide for you: