As mentioned above, You can move, you can attack, but you can not cast a new spell.
Normally a spell with a (D) requires an action, and since actions (with the exception of speech as a free action or IMMEDIATE actions) can only be performed on your turn would suggest you have to wait until your turn.
HOWEVER, since dismissing a spell that you yourself are concentrating on to maintain, does NOT require an action (read further down for more details on that), it indicates that it does NOT require your turn to dismiss a concentration(D) spell.
Conversely, swinging again to the other side, the specific entry on (D) states "ON your turn". I would honestly present all of these points to the GM and have a house ruling on it.
Personally, I would rule that you could dismiss a spell with verbal components (a quick sentence=free action even out of turn) at any time, and spells that require somatic components (a quick gesture=free action on your turn) for you to wait until your turn. That way, the complexity of the spell determines how easy and quick it is to dismiss.
I'd also love to comment on SweetSatu & YogoZuno, but without the "required 50 points" I can't contribute without leaving this as part of my answer. Blame the system. Anyways...
Not to mention, the information is incorrect. Dismissing a (D) spell is actually a
Standard Action...so, it is, in fact, less effort to stop concentrating than it is to
actively Dismiss the spell. – YogoZuno Feb 7 '18 at 20:42
The VERY next line of that same entry you quoted ALSO states:
"A spell that depends on concentration is dismissible by its very nature, and dismissing it does not take an action, since all you have to do to end the spell is to stop concentrating on your turn."
So a (D)Dismissible spell that requires Concentration to maintain, does NOT actually require a standard action to dismiss it.