How do I alter the payment terms so an invoice due date is 14days rather than a month?

Hello Harriet_brown1-h, 

Welcome to the Community page, 

If you click on the main cog wheel top right hand corner> click on account and settings> sales> sales form content>preferred invoice terms>click on the drop down arrow> click on add new> name it what ever you like for example net 14> in the box where it says days you would put 14> click on save> and save again.  When you go to create an invoice click on the terms box drop down arrow and select the new one you have just created so the 14 day one for example. You can create as many as you like as well just to let you know.