Unity actually has the capability to create and edit animations. It's not as powerful or easy to use as a 3D Modeling package's abilities but it is available should you need it for any reason. For example, if you wish to slide a texture around to create some sort of effect without the need to script it, you can do it inside Unity.
To open the Animation View, go to Window -> Animation.
For an extremely in-depth guide on Animation View: http://unity3d.com/support/documentation/Manual/Animation.html
Modifying an Animation created in a 3D Modeling package
Animations imported from a 3D Modeling package are set to read-only.
In order to modify it, a duplicate copy must be made. To do this, select the .anim file in the Project Panel (it will have a clock icon) and press ctrl+d. Rename the duplicate and re-assign it to the model.
This duplicate may be modified.