I see that this idea has been suggested and closed many times. I have several paid accounts with dropbox including my personal and business accounts. We really need this simple option that has been around for decades!
When we go to search our product images throughout months, then we cannot deselect. We will need to close the tab or start unselecting each photo. There is no option to unselect all or clear selection to start a new batch.
It seems to be so trivial, and every single app has this feature. But every time it has been proposed by users, it has been closed.