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

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

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.