While collection assets themselves aren't editable (other than basic resizing), you can access the original collection which will contain the actual meshes and materials which you can edit. To do so, select the object (collection instance), press CTRL+A, and click the "Make Instances Real" option.

The ceiling fan's light material is controlled by a spherical gradient texture that uses object coordinates. This means that the textures object mapping will need to be adjusted in accordance with the height of the ceiling fan. There is a value node in the light's material which lets you easily adjust this which should fix the problem.