1.Go to your repository.
2.Click on "Add File" and "Create new file" Button.
3.Type the file name as License.txt or License.md in the input box next to
your repository name, a drop down button appears towards right side.
4.Choose the type of license of your choice.
5.Click "Review and Submit" button at the right side.
6.Choose "Commit directly to the main branch."
5.Click "Commit new file" button at the bottom (Green button)