Drag the exported photos into the 'Photos' library or you can click 'File' and then select 'Import to select' option to import them.

Choose a destination and click 'Ok' button.

Choose 'Original' or 'Current' option from 'Kind' pop up menu.

Select all photos and choose 'Export' option.Steps to Export Photos from iPhoto Library: Choose 'Rebuild Database' ooption to repair the iPhoto Library. To do so, launch 'iPhoto' and hold down 'Option' to select a library, then hold down Command and Option when clicking 'Choose' button. If you have trouble with Photos app stalling during an import, then you can rebuild your iPhoto Library. To do so, hold down 'Option' key when launching the program. But, you can create a separate Photo library and import your second library into that. That means, if you choose iPhoto Library while launching Photos app, then you can not import an Aperture library into the Photos library. Once you choose your library, you can not import another library into Photos. When you first time launch Photos app, it will ask you to choose which library would like to import into Photos app.

Combine mac photo libraries