Editing is not allowed on batches with a status of "Closed."

Click on "Contributions > Batches" from the left-hand navigation menu. Then locate the batch you want to edit and click on "Edit" at the end of the row. When the batch opens, click on "Edit batch details" from the "Actions" menu in the top right.

See this article for more information on the editable batch fields. See this article for details on how to add transactions to batches.