The credentials of thousands of Apple users were stolen in the hack / Sean Gallup/Getty Images

Days after the update is released, 16 per cent of people have upgraded — and despite some slow downloads, Apple seems largely to have stemmed problems that have hit previous upgrades

More than 16 per cent of people have already upgraded to iOS 9 — leading to some reports of early problems with the software, but none of the difficulties that have plagued previous releases.

With around 1 billion iOS devices in the world, according to estimates, that means that 160 million upgraded to the software in its first day and a half, according to Mixpanel, an analytics company.

The huge number of people downloading the software did initially mean that updates were slow and were sometimes being thrown off by server problems. But days after the the new update was released, the download seemed to have sped up and was available within 10-15 minutes of requesting it.

There were some reports on Friday that issues were also hitting those who had managed to upgrade, particularly people with older devices. A Reuters report suggested that some users were being locked out of their phones and so had to reset them, losing information that hadn’t been backed up.

The issues seemed to be isolated and reports of lasting problems were minimal.

Those difficulties do seem to be fixed by resetting the phone. But Apple engineers were working to find a fix that didn’t lead to the loss of data, Reuters reported.

All users are recommended to fully backup their phones and their data before upgrading to the new operating system. While most upgrades seem to be going ahead without problems, backing up means that all information can be retrieved if phones need to be reset.

Last year’s iOS 8 upgrade was hit by many more problems than the new operating system. Many had difficulties installing it because of the huge amount of space that it required to install, for instance, and then the first version brought problems for some including difficulty connecting to WiFi.