Select all elements on the board using Ctrl+A / Cmd+A shortcut
First of all, make sure you've chosen Select on the left-side toolbar. Then hold down Shift (skip this step if you use the touchpad), click the board and drag the selection field across the objects as shown in the animation below.
Selecting and moving three stickers at once
Another way to select multiple objects is to click them one by one while holding down Shift.
The selected objects can be moved and resized together - just click one of the objects within the selection field and drag! You can also lock, group and align them using the options in the menu.
Make sure none of the objects you want to select is locked. Otherwise, you'll want to unlock them first by clicking the object unlock:
Unlocking an object on the board