I seem to have a problem uploading blocks. After carefully selecting all the different stages of uploading, the block did not load correctly. I read through the help file, and there it specifies that the block uploaded must have a preview screen, my attempt at that was unsuccessful. Please give me recommendations. Secondly, how can I update the uploaded block, after correcting the problem?

This particular upload has got stuck at the file system level - probably a failed upload or a very old (unsupported) Inventor version. We have updated the uploaded file manually.
 
In a standard situation, you can update the block by uploading the same file in the same category (by the same uploader) - check the Overwrite option.