Currently, you cannot Undo a categorization but you can change it.
1. Go to Search tab and search for the transaction you would like to re-categorize by entering a payee name into the search field.
2. Tap Edit on the upper right corner and check transaction you'd like to change,
3. Then tap Categorize Transaction.
4. Select new category.
Note that you can also select and re-categorize multiple transactions at once.