I want to switch to Ubuntu devel sources. What is the best way to switch to it??
By "best", I meant what else should I do before or after editing the apt sources file.
An overview of switching to the development (pre-release) of Ubuntu. It's not detailed simply because most folks with sufficient skill and experience to use development version of Ubuntu already can locate (or already know) more detail.
Skills: Testers and developers are expected to know how to troubleshoot, use the bug reporting system effectively, locate and use the published documentation, use apt and virtual machines to accomplish their goals, and other skills.
Communicate: Join the pre-release discussion and announcement mailing lists and testing online groups.
To actually begin testing or developing software, the usual method is to spin up a virtual machine and install the pre-release daily .iso image.
For switching on bare metal (warning: It's a one-way trip. You cannot 'switch back'), plain old do-release-update -d without editing apt sources is the usual way. This method is disabled during the weeks after a new release until the next +1 repos are set up. There are other methods, too.