How can I change the payment terms text?
The payment terms are defined in Settings -> Document Options -> Payment Terms. Here you can add, modify or delete the payment terms.
The content in the Text field is what will show up on the invoice. Note that changes here will not immediately take effect in an existing document, since all changes are stored as a new version. That is to make sure that an existing invoice does not change without you wanting it to.
To apply the new changes, simply select the option without the * (the * indicates that an expired version).