CANT UPDATED MAC.

Hello,

I am having a difficult time updating my Mac. It is a Mac air that I got in 2014. When I look at systems it is running OS X Yosemite 10.10.5. I am not sure the last time it updated. However, now all the streaming sites say the browser is no longer supported without an update. My computer doesnt say that its due for an update on the system preferences and when I try to download an OS from the apple site is says this...

"This update is not available for this Apple ID either because it was bought by a different user or the item was refunded or cancelled. "

I recently changed my password to my apple ID but I have tried logging out and back in to the store.

Please helppppp.

Thank you.