Can I use YouTube transcript data in my blog? Or do I have to make some changes before any blog post? I am unable to understand what is wrong or what is right. Please explain simply.
That depends very much on what you mean by "use youtube transcript data". It may also depend on what jurisdiction you are in, in this case what country you are in.
In general, one cannot simply copy the words of a youtube video and use them in one's own blog or other publication without permission. Those words are protected by copyright. Limited quotes may perhaps be made under fair use, or fair dealing or another exception to copyright, but the rules will vary depending on the country. Such quotes must usually be limited in both purpose and extent. See threads on this site tagged fair-use for more details.
The ideas discussed and any information presented in the video are not protected by copyright. Thus if a youtube video reports that Joe Smirt was elected Sheriff of Capsicola, you can mention that fact on a blog. But if the video describes the election as "the most wonderful and surprising upset in over a century, leading to a new day for Capsicola" those words are protected, and you should not copy or closely imitate them, unless in a clear quotation, attributed to the author of the video or to whoever actually said them, if that was another person.
It is generally not enough to make minor changes in the words of the source, such as to replace "surprising" with "astonishing". One must rewrite the information in one's own words.