How to extend a final note

Hi everybody,

I'm editing a short intro for my YouTube Channel, and I've edited a proper audio track modifying a song so that it end in 10 seconds, with the correct beats.

The problem is that, at 9.05 seconds, there's a very low note that suddenly stops, with no fade: I need to extend that note for other 0.25 seconds, sot that it progressively lowers in highness.

How can I do that? Is there an effect I have to activate, a plugin to use ore sonething like that?

Thanks in advance for your support.

Greetings.

MoroX