You can make changes to kernel sources in VSCode and play with the modified kernel.
On making changes to the kernel sources it's necessary to rebuild the kernel for the changes to take effect.
Building the kernel is setup as the default build task. You can use the build shortcut Ctrl + Shift + B to automatically start the kernel build.
You can also start the build from the command palette by typing:
Tasks: Run Build Task
Once you rebuild the kernel, also Rebuild the fs-image.
Now, you can restart and debug kernel.