To select multiple tasks on the web or desktop app, hold down "Alt" when clicking on the tasks you want to select. Once selected, you'll see an "X selected" button appear in the top right hand corner of the list. Click this to open the bulk actions menu.
On the mobile app, tap the "Edit" button shown at the bottom right hand corner of the list and tap on the tasks you want. You'll see the buttons at the bottom change to "Move", "Archive" and "More".
From this menu, you can move or copy multiple tasks to another list, or duplicate or archive all selected tasks. The bulk action option allows you to make certain changes to multiple tasks at once.
When you click "Bulk action" from the menu, a popup appears. Each of the four tabs lets you set, add, remove or replace certain values, depending on the field you choose to change.
The only fields you won't be able to change in this way are file fields. Files need to be added or removed from items on an individual basis.
"Set" sets the value of a field. This means that it will overwrite any previously set value. It works in a slightly different way for each of the field types.
Add" allows you to add more information to data that already exists in a field. It will not overwrite the existing field, just add to it. This option isn't available for all fields.
For most fields, remove works as a "find and delete" function. It will not remove all data from the fields, just the part of it that you specify. This option is not available for all fields.
This can also be thought of as 'search and replace', and works for every field type.
The "Field" section allows you to choose which field you want to make the change in, then the "Value to set/add/remove/replace" field is where you pick the new value for that field.