That is the most useless answer ever to such a question.
A sky in Half-Life consists of six .tga files wiht a prefix in the name for the position:
the top is up_namesky
the botton is dn_namesky
etc.
You can select the sky you want in the map properties tab. Here is a lost wiht all the skynames and the tutorial about the subject at The Whole Half-Life.
http://www.twhl.co.za/tutorial.php?id=6Next time do a search wihtin the tutorials on this subject.