iPhone6 restore from itunes

I backed up my wife’s iPhone 6s to iTunes, because her phone got wet and needed replacing.

After getting her replacement phone, I am trying to restore.

We have an older Mac Notebook.

when trying to restore, its telling me that I need to upgrade to iTunes 12.8. My computer doesn’t have any upgrades listed. I found iTunes 12.8 and downloaded. When I open the file it says it’s damaged and should be put into the trash. I’ve tried this at least 10 times with the same result. Please help.

thanks

nat