If you want to change a category or add a category to uncategorized transaction please proceed as follows: 

iOS and Android Version:

• Via Settings:
  Settings > Categories & Rules > scroll down to 'show uncategorized transactions'
  swipe right to add a category
  If you select 'Edit' in the upper right corner you can mark several transactions that shall get the same category > tap on the category symbol and select the category or add a new one.

• Via transaction details:  
  Open the transaction details in the transaction list and change the category or add a new one
  or
  swipe right on the transaction in the transaction list, to change the category or add a new one.

macOS-Version:

• Via Settings:
  Options or Settings > Categories & Rules > show uncategorized transactions > click on the transaction and select a category 

• Via transaction list / transaction details:  
  Open the transaction details in the transaction list and change the category or add a new one.
  Press and hold cmd to mark several transactions, that shall get the same category.
  Tap on the category symbol and select the category or add a new one.