To update multiple tasks/projects on the mobile app in bulk, tap the ⋮ icon at the top right, and there you'll see the "Bulk Edit" button. After you tap it, you can start selecting items that you want to edit.
To edit the selected items, tap an icon in the bulk edit drawer at the bottom of your screen, or tap the ⋮ icon on the right to choose from all available actions.
If you want quicker access to Bulk Edit, you can make Bulk Edit option the default header action. To do that, tap and hold the icon next to the ⋮ icon.
Something worth mentioning is that the buttons/icons you see when you open the Bulk Edit are the same actions that you can set using the Quick Edit Actions option in the settings.