I had saved a document of 39 pages. When i shut down and restarted my system and launched word, i was prompted to save "Autosaved" version of the same document. I saved and found out it was an older version with 19 pages which replaced the new version. PLEASE, HOW CAN I RECOVER THE CURRENT VERSION OF 39 PAGES?

You have overwritten your 39 page 'Current' document with the earlier 19 page document and I don't think there is way to get the 39 page document back unless, by pure luck, there is a restore point on your computer that is dated yesterday after you saved your 39 page document. You might get lucky so check your restore points.

It will be interesting if someone else can show how to retrieve a previous version of a document since it will help others.

The most effective method I know of to get rid of a single computer file is to save a different file with the same name in the same location. Sorry.
What you are talking about is not AutoSave, but rather AutoRecover. See Automatically backup Word documents by Graham Mayor, MVP.