You can use an internet browser on any device and log into www.icloud.com and go to the Photos app there to access iCloud Photos (note that this refers to iCloud Photos, not to iCloud Backup).
The following may also be of interest if you want to transfer them to Google Photos: https://support.google.com/photos/answer/10502587?hl=en
After you no longer need your upgraded iCloud Storage plan, see the following to downgrade it back to the free 5GHB plan: Downgrade or cancel your iCloud storage plan - Apple Support
If you don't have an Apple device or a Windows PC, see the following from that same site:
If you don't have one of these devices, contact Apple Support to change your iCloud storage plan.