If you want to manual update then click here to download Generate Thumbnail
Go to Dashboard>>Plugins
Click on Add New Plugins
Click on Upload Plugins
Final step is upload and activate it.
If you want to install Generate Thumbnail Then type themeegg find the Generate Thumbnail
It is use ful when you are going to add any images on your pages or post you can easily create Thumbnail Image with different sizes.
Content Area Demo
Generate Thumbnail Plugin