The files are downloaded as compressed .tar files and can be opened using zip utility such as WinZip / 7zip.
Assuming you've downloaded 7zip and are using Windows, please go to
- Start> Computer > Downloads
- Select the downloaded file
- Right click > Select 7 zip > Open archive
- Access the file directory and select one of the files
- Select the file you'd like to open
- Right click & select 'Open With'
- Select 'Notepad'
The data will then be available in Notepad in .json format. You can then use other tools to view the data in the format of your choice.
There’s sample code available via the http://docs.developer.betfair.com/docs/display/1smk3cen4v3lu3yomq5qye0ni/Sample+Code#SampleCode-HistoricalData for processing the new files and also a blog post via https://underround.wordpress.com/ regarding processing the BASIC files for analysis using Python.
If you prefer to do this manually, you can download he BASIC data files into Notepad (see instructions via https://historicdata.betfair.com/#/help) and then use an JSON to CSV conversion tool. This will create relevant column headings with Excel and enable to you plot the last traded price over time for each selection.
Please see some JSON to CSV conversion tools via https://www.google.co.uk/search?q=json+to+csv&oq=JSON+TO+&aqs=chrome.0.0j69i60j69i65l2j69i57j69i60.2734j0j7&sourceid=chrome&ie=UTF-8