You can make edits to entries in bulk using the multi-select tool in Day One. For version 2.7+, swipe on any entry and tap the Checkmark icon to begin multi-select.
Once multi-select is enabled, tap the button to the left of each entry to include in the bulk actions. At the bottom there are options to Move to a different journal, Tag, Add/edit location, Delete, or Share selected entries.
You can also tap Select All to select all entries in the specific view.
Tap the More button (•••) to show options to Edit the entry, add as a favorite, move the entry, or share the entry.