iOS 8 can be installed for free on all of the iOS devices that are supported for Salesforce1 use. For help upgrading devices, see Update the iOS software on your iPhone, iPad, and iPod touch on the Apple Support site. For the full list of iOS devices that Salesforce1 supports, see Requirements for the Salesforce1 Mobile App.
While users can continue to use previously installed, older versions of Salesforce1 on devices running iOS 7, Salesforce will no longer provide support for any issues encountered.
With Summer ’15, the Salesforce1 mobile browser app is supported on devices running iOS 8 or later. Users aren’t prevented from accessing the mobile browser app on iOS 7 devices but may encounter performance or other issues that won’t be addressed by Salesforce.