How to view backup files as asking what app to use

WD External and Portable SSD External SSD for Windows
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I have just bought and set up a WD My Passport Portable Hard Drive. I have run a backup, which seems to have worked. To check I wanted to open a file to see it. I can see the files but if i click to open one, especially in Excel, it asks me what app I want to use to view it and won’t give me Excel as an option even though it is on my pc. So how do I see it?