ZOHO CRM - changing field properties

ZOHO CRM - changing field properties

Hello, please would you let me know how you change a field's properties without having to create a new field and delete the old one - moving all the data across or reenterring?
e.g. If we originally set up a field as 20 chars and want to change it to be 30 chars, can we do this? I can't see a way?

Many Thanks
Bill