If your business gives customers a set number of credit days then you can set up a default value by clicking 'Your Business' > 'Company Details'.
Scroll to the bottom and select the number of credit days you want to give customers by default.
Note: The default credit days are applied when a customer is created. If the default credit days is changed at any time, for example, from 14 days to 30 days credit then this will only take effect for all future customers.
It will not change existing customers that have a default of 14 days credit to 30 days. Those customers will have to be changed manually.
An individual customer's credit days can be changed by searching for the customer and manually editing their customer credit day record.