Had a software issue with my iPhone 5. Went to apple today and they replaced my phone. They told me to restore from iCloud using the computer. My Mac is having issues so I can't use it. Plugged it into the families PC and it wouldn't show the recent update(backed it up at the store to iCloud). So I reset my iphone and tried to restore from the latest backup but since it doesn't have the newest IOS I can't update it. How do I fix this?