Adobe Audition: how to replace track with higher quality version

Partly, it depends upon why it is a different length, and also what sort of track it is. If it's singing, you have to be a lot more careful. But the options are:

Stretching or shrinking it until it fits in the timeline. Alternatively, if you want to make words fit really accurately, you could use the Automatic Speech Alignment tool in the Clip menu. Or, you could just break the replacement clip up into phrases and shift them individually so that they are where they need to be, without altering the precise phrasing.

But really, it depends entirely upon what it is to start with, and exactly what you want the result to be.