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
The final step is upload and activates it.
If you want to install Generate Thumbnail Then type Themeegg find the Generate Thumbnail
It is useful when you are going to add any images to your pages or post you can easily create Thumbnail Image with different sizes.
Content Area Demo
Generate Thumbnail Plugin