how do I upload photos from a shared album to a pc?

From the article I previously linked:

Use these steps to copy content from a shared album in iOS 8 or later:

1. Open Photos, tap Shared > Sharing, then tap the name of the shared album.
2. Save photos using one of these methods:
   • Tap an image to open it, then tap . To select more images, scroll the images left or right and tap the ones you want to save. Tap Save Image.
   • Tap Select, then tap the images you want to save. Tap , then tap Save Image.
3. Import the photos to your computer.

You will have to save those items to your camera roll first. You can not import photos or videos to a computer from anywhere except the camera roll.

This article may help:

iCloud Photo Sharing - Apple Support