At the top of the Enter Coins page you can delete transactions individually, grouped or completely.
If you select just one transaction by ticking the box, a delete button appears.
If you click on "Bulk Edit/Delete", you can delete all transactions as well as bigger groups of transactions, e.g. selected exchanges or time frames
Here is an example for an exchange:
Alternatively you can delete imported transations by date: either "transaction date" and/or "import/add date" (which is shown on the trade list (full view)):
You can also select and delete multiple transactions at once in the table on the Enter Coins page using the SHIFT (range select) or CTRL (single select) key on your keyboard. This however is limited to 500 transactions at a time (no matter how many you highlight).