If you want to set default payment terms for a specific contact, e.g. 30 days, you've come to the right place.
Please note that all future invoices for that contact will default to those payments terms - and if you want to change a specific invoice, you'll have to edit its payment terms manually.
Navigate to the Contacts page, and select edit on the appropriate contact.
Scroll down to Invoicing Options and enter the number of days, e.g. 60.
Select 'Save' at the bottom of the page and your contact details have now been updated. These payment terms will be used on all new invoices and new recurring invoice profiles that you create for this contact.
Existing invoices and recurring invoice profiles
When you update payment terms for a contact, any existing invoices (including draft invoices) will not be changed. If you want to update any existing invoices, you'll need to edit these manually.
If you have any existing recurring invoice profiles set up for this contact, these will also remain unchanged. If you want to update the payment terms for a recurring invoice, you can edit the recurring invoice profile.