How do I switch photo libraries?

Hold the option key on launch. More details... Designate a System Photo Library in Photos - Apple Support

How do I switch photo libraries?

You have been reading about migrating an iPhoto Library, when you posted this question.

Do you want to switch between your old iPhoto Library and the new Photos Library?

Or is the problem, that you cannot switch between different Photos Libraries, perhaps a library on an external drive?

Quit photos and

1 - double click on the library you want to switrch to, or

2 - drag the library you want to switch to to the Photos icon in the Dock, or

3 - hold down the option key while launching Photos and use the resulting select library window to open teh library you want to switch to

LN