Sorry, in current versions of modeling mode we no longer allow you to edit the default engine geometry, but in 4.25 we had not added that yet. You cannot re-import this asset because you don’t have the source fbx/etc it was imported from. These default shapes are just included inside the Engine in binary form.
The default cube asset will be in your engine install directory, eg
C:(YOUR_ENGINE_PATH_HERE)\Engine\Content\BasicShapes\Cube.uasset
The problem is you need the (unmodified) version of this file. The simplest way to do this is unfortunately to either re-install the engine, or make a new 4.25 installation, and copy this file over.
A less drastic way is to use the Github version of the engine temporarily. If you do steps 1 and 3 of the engine setup in the Readme here (you will need a github account):
https://github.com/EpicGames/UnrealEngine/tree/4.25
This will involve downloading a zip file of the engine source and then running Setup.bat, this will download the Eingine\Content\BasicShapes*.uasset files, and you can copy Cube.uasset over into your engine installation.
(sorry for the trouble, and that there isn’t an easier way to fix this)