There's a cheat sheet [en.wikipedia.org] as well as a tutorial [en.wikipedia.org] that explain the formatting. If you're just updating a dead link, locate the URL in the editor, change it, briefly explain the change you made, and save your changes.

Hi @robzilla. Thanks for your great support but still I've confusion and not clear about code in the editor window there are already some code available.

My question is suppose in the reference there are 3 links but 1 is dead among them. If I try to edit dead link, I've to go and click on reference edit. The editor window opens with inbuilt code without that two reference links.

What to do I don't have any idea?

The "References" list is automatically generated based on references used throughout the article, and so clicking [edit] next to the "References" header will not actually show any code for those links. On the article page, click the "Edit" tab at the top right to get the full text.

Thanks for letting me know @robzilla. Let me do the way you have said and just see what happens.

Possible, it would be better to ask some of elder Wiki user to fix your problem in to prevent wasting time.