May I change a field name for a few modules in the Free Edition?

May I change a field name for a few modules in the Free Edition?

I want to use Zoho CRM for a charity. I am tracking donors and donations.  So I have relabeled my modules to reflection donors instead of contacts and leads are now potential donors. Opportunities are donations. I want to set up an API to connect to donate.ly where we actually receive and manage our contributions. they have a donor ID field that I would like to put into the Assistant field. So I am hoping I can relabel the Assistant field (in the Contacts Module) to be Donor ID where I would store a unique combination of alphanumeric characters.  

I cannot find a way to change the labels of fields in the free edition. Did that option disappear when I converted from a trial to a free edition or am I just missing a very elementary step? 

Please help.

Thank you.