With Enscape 3.5, we introduced the multi-asset editing feature, which allows you to select all instances of a placed asset. Please note that this feature is not available in Enscape 3.4 or older and currently only available on Enscape for Windows.
In the Enscape window, make sure that the Asset Library is open (by clicking the button in the toolbar or by pressing “L” on your keyboard. Find one instance of the asset you would like to select and right-click it. This will open a menu which allows you to use the option “Select All”.