You need to purchase for Lr 5.7 as it is an upgrade .
You may purchase it from the link : http://www.adobe.com/products/catalog/software._sl_id-contentfilter_sl_catalog_sl_software _sl_mostpopular.html
Lightroom 5.7.1 is a paid upgraded application.
You can purchase Lightroom 6 (latest version) for your computer.
Hope this helps.
Moving from Lightroom 3.6 to Lightroom 5.x or Lightroom 6.x is an upgrade that must be purchased. Lightroom 3.6 was the last version of Lightroom 3 that you would receive at no additional charge. Lightroom 5 will have to be purchased from a third-party vendor because Adobe doesn't sell it anymore. But you might as well just moved to Lightroom 6 or Lightroom CC. You don't need to do incremental upgrades. You can go directly from Lightroom 3.6 to the current version.