The edit-compile-test loop is the quickest way so can test your changes.
1. Go to the package's source code folder and edit your files.
2. Compile that particular package, e.g.
bitbake linux-imx -c compile -f
3. Deploy, e.g.
bitbake linux-imx -c deploy
4. Insert the SD card into your host, and copy the Linux kernel into the the first partition
sudo cp tmp/deploy/images/uImage /media/Boot
5. Insert the SD card into your board, boot and test your changes
6. In case you are satisfied with the results, you need to create a patch and add into the package's recipe. Check this document to execute this step.