To switch between libraries
- quit Photos, if it is running.
- Then launch Photos again while holding down the options key ⌥. keep holding down the ⌥ key firmly, until you are seeing the dialog to select a library. Now click the library you want to open in the last of libraries.
If the library you want to open does not appear in the list, check, if the library is in a supported location. Photos will not open a library on an unsupported volume, like a network share or a Time Machine drive. If the library is on an incompatible volume, move it to a volume, that has been prepared as described here: Move your Photos library to save space on your Mac - Apple Support Your Pictures folder would be the best place, if you have enough free storage.
If the iPhoto Library cannot be opened at all, check the filename extension. if the extension is ".migratedphotolibrary" because you previously opened the library already in Photos, change the filename extension back to ".photolibrary"
Then Photos can open it again and create a new Photos Library from it.