These are simply two forms of the same word. However, because one is plural and one is singular, you cannot interchange them. Addenda is the plural form, and means additions, especially to a book or other written document. Addendum is the singular form of addenda.

What is the correct plural of addendum?

noun. ad·​den·​dum | \ ə-ˈden-dəm \ plural addenda\ ə-​ˈden-​də \ also addendums.

What are addendums?

An addendum is an attachment to a contract that modifies the terms and conditions of the original contract. Addendums are used to efficiently update the terms or conditions of many types of contracts.