Yosemite doesn't want to install for me
Okay, so for over quite some time, I've been wanting to download Yosemite in my iMac, however, every time I download it, it keeps on saying " This copy of the Install OS X Yosemite application is damaged, and can't be used to install OS X." Then it kept on saying to delete that copy and get another one, so I did that. It still keeps on telling me I got a damaged copy of it. I finally gave up, and came here.
Please help me on this situation, if you need any information about my iMac please do ask.
iMac, OS X Mavericks (10.9.5)