Click the Bulk Delete Chats icon in the toolbar (pin it from the puzzle-piece menu first), or press AltShiftD. On a Mac, Alt is ⌥ Option.
Tick the chats to remove. Search by title, shift-click to select a range, or use Select all.
Choose Archive or Delete and confirm the count.
Not sure? Archive first. Archived chats can be restored in ChatGPT under Settings › Data controls › Archived chats. Deleted chats can't be brought back.