Here’s how you can edit the "Terms & Conditions" in an invoice/estimate:
1. Click the Invoices for editing an invoice and the Estimates tab for editing an estimate.
2. Edit an invoice/estimate by clicking the edit sub-link that appears when you mouse-over on the list.
3. Scroll down and make the required changes to the Terms & Conditions.
4. Click and select the option “Set this as default".
5. Once this is done, click the Save button.
Zoho Invoice FAQ