When you use the website, just click on the note’s header - where the note title is situated. You will see indication that the note is selected. Deselection happens if you click the back arrow under the NotezApp logo or if you click the selected note’s header again.
In the mobile app the note selection happens with a long tap on the note in the note list. The note is selected when you see big check mark in the middle.
On the website it is the same as selecting single note. Just click the headers of as many notes as you want to select.
In the mobile app after you have selected a single note with a long tap on it, every next note can be selected with a short tap.