How can I access my iPhoto library now that my Mac has automatically upgraded to 64-bit apps?

One more thing to consider - your iPhoto Library needs to be on a compatible volume, if you want to migrate it to Photos, because Photos will create the new Photos Library on the same volume. You will need some extra free storage on the volume as well. If the iPhoto Library is in your Pictures folder, the format of the drive will be no problem, but if the Photos Library is on an external volume, it should have been prepared as described here: Move your Photos library to save space on your Mac - Apple Support

Hi

You can upgrade the library for the photos app, by holding down the option key while starting photos. You will get the library chooser pane (see screenshot). Select your iPhoto library and open it, and photos will then upgrade it (It may take some time for a large library). If it is not listed, click "other library" and select it from the finder type pane.

Note that Photos does not work exactly the same as iPhoto so there will be some differences (eg. Events will be converted to Albums)

It is a good idea to make sure you have a backup first because the upgrade can (rarely) go wrong. If you already have it in time machine - that will be fine as a backup.