Again, try to keep generic questions about android devices out of this rom-specific thread, this is not the place to learn about the basics, this is a rom provided for those who already have the knowhow/experience to use them.
All flashing/device modifications are done by you, at your own risk, with your own knowledge. All of this information is available not only on XDA, but also easy to find on Google.
That being said, I'll answer this time.
You can either reboot to bootloader after flashing one, and do
depending on which slot you need to switch to
Or you can use the method here: