how to learn reverse engineering in catia v5?

You cannot "learn" reverse engineering You Have to try and decide how the Product is made and then try to replicate it to the best of your skills.

If you want to reverse engineer a product that is one thing. If you what to know the process it is another.
From what I was taught the process goes as follows:
1) You must document everything that you do.
2) Use the product and see how it functions and what other products in the market that do the same function.
3) Create a CAD model of the product.
- Do not break any part that you can not take apart.
- After you documented all the prosses and have all the CAD parts that you could take apart. Break the remaining parts and see what type of adhesive was used.
4) After completing the CAD assembly create a product report that states what parts were designed for what users.
5) Also, determine the materials that were used and the production process for each part.

After all of that, you should have a nice understanding of what it is to reverse engineer a product. The best way to describe the process is "You need to enter the mind of the team that created this product and read it".