Updating an old app that was installed under another account.

You can't even if you had the account for it. Apple discontinued iPhoto 9 years ago. There is no actual update for iPhoto anymore.

What you can do, is open the iPhoto Library with Photos, by holding down the option key when opening Photos from the Applications folder in Finder and navigating to the iPhoto Library.iphotolibrary file you wish to open and see if Photos can convert and import it correctly.

Here's a bit of stuff from Apple about importing pictures from one Library into another, like Phil0124 suggested. It doesn't mention iPhoto, but it's the same process:

Import photos from another library in Photos on Mac - Apple Support

This imports the pictures from the other Library, but not the events, folders, & album organization. If lots of time was spent in organizing that Library, and you want to keep those events and such, then the only way to do that is with the trusted 3rd party app PowerPhotos ($35.) This app does lots of other helpful stuff, too.

If you don't care about all that previous organization, then just use File>Import, and it should work well.