If you need for any reason a clean slate with IJC, a clean re-installation can be performed. It is a quite simple process.
Locate your IJC User Directory - its location can be found in the "About Instant JChem" dialog, which is accessible in IJC's Help menu.
Shut down your IJC and perform an uninstallation normally, according to your operating system.
Delete your IJC User Directory - the directory itself is named 14.9.1.0 , but the complete path will differ depending on your operating system.
{info} Mac OS X : /Users/<USERNAME>/Library/Application Support/instantjchem/14.9.1.0
{info} Windows : C:\Users\<USERNAME>\AppData\Roaming\.instantjchem\14.9.1.0
Download desired IJC version installer from the web, if not already downloaded.
Install IJC normally.
{primary} Please note that your data will be intact as IJC Projects are stored in a different folder. After the clean re-install, IJC will not remember your last opened projects, so you have to open them again.