How can I change my choice of program to download to?

I tried to download my ebook from KOBO's site but when I tried to open it in Powerpoint I got the error message:

"Powerpoint can't open this file. The file may be not recognized by Powerpoint or the file extension may not match the internal format of the file."

So I double-click the download to try and change my choice from Powerpoint to try say WORD for example but it now thinks I only and forever want to read that file in Powerpoint and it pops straight up with the Powerpoint message every time I retry so it seems my first incorrect choice of program to try and read the file can't be changed - ever.

What can I do to read my ebook in some program other than Powerpoint when it keeps assuming that I only ever want to use my first choice which in this case was Powerpoint?

The file extension is .epub and that seems to require a special reader which I'm hesitant to download off the internet from sites I don't know anything about.

Can anyone please offer some way around this problem?

I realize it's sort of two questions in one and I'm sorry for that but it's got me flummoxed.

Thank you!