I have installed an addon suppose named some_addon where there is a block folder named some_block.

Now to customize the block.

Make a new my_template folder in your block's templates/ folder.
Copy the file view.php from its original location up to blocks/some_block/templates/my_template/. If that block also has a folder called js/, copy js/ up to blocks/some_block/templates/my_template/
make changes in your copy of view.php

But this is showing me an error in blocks edit mode that my form_setup.html file is missing. Please help how to do that.