Your content is stored on the filesystem with the location pointed to by the repository.properties.
In particular the settings for dir.root, dir.contentstore and dir.indexes. Which should be fairly self-explanatory.
So to move your content store to a NAS what you would do is
A) Mount your NAS device on your alfresco server. On windows this will show up as a drive letter. On unix will be part of a filesystem.
B) Shutdown alfresco.
C) Copy your content store to your NAS device
D) Change your custom-repository properties to point to your content store on your NAS device.
E) Start up and test alfresco.