Alternative update + Magento update
If you are using WordPress, you should go to your Plugin's list, deactivate and delete the Nextend and Smart Slider 2 plugins.
If you are using Magento, go to your Magento Connect Manager, and delete the NextendSmartSlider2 and Nextend extensions.
If you are using Joomla, you don't need to do this step.
Install Smart Slider 2 the same way, as you did it the first time. Your sliders will stay.
Go to the Global settings -> Cache, and click on Refresh Slider.