Update InfoPath form field value in SharePoint Online document library?

If you are using buttons as Submit and Save, you should modify your Submit Data Connection.

First create a field that will handle the File name of your form. Name this FileName. Create a action rule either on form load or Submit button. Take note that put in the Condition of the rule that if the field is blank then produce a file name value (It can be any format that you want). This condition will eliminate the creation of a new file name value after submission.

Now go to your Submit Data Connection, put in the FileName field under File Name and check the "Allow overwrite if file exists".

Put a rule in Submit and Save button with your Submit Data Connection. The idea is that when creating new form, the FileName is still blank and still not saved. After submission, the FileName will not produce a new one but use the saved value. When Save button is pressed, it will overwrite the existing form

There are two button in Ribbon of InfoPath Form Saveand Submit.

Submit button will always create a new list item in the document library whereas Save button will update the existing form.

So, for the first time submission you should use Submit button. Later, if any update is required in the existing form, click on Save button instead of Submit otherwise a new list item will be created.