With bulk-edit, you can modify many transactions at once! In order to access it, click on the three-dotted options button next to search box and select Bulk edit.
You'll be presented with a modified interface with round check-boxes on the left and the following bar at the top:
As you click on transactions, you'll notice that they get included in the selection. If you hold Shift key on your keyboard while clicking on a transaction, it'll select this transaction and all transactions in between of the previously selected one.
Please note that bulk-edit has the following limitations. Remember that they only apply to editing transactions but don't apply to non-edit actions such as deletion or setting as cleared:
you can only bulk-edit transactions of the same type: only expenses or only incomes,
it is not possible to bulk-edit transfer transactions or unique transaction types: adjust balance and refunds.