Hi khairiaty, welcome to the forum!
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You do not need to uninstall the Little Reader trial software, especially if you are already using the latest version. All you'll need to do once you get your license key (either from your account page / receipt or when you receive your package) is to import it to activate Little Reader.
Here are some articles with instructions on how to import your license key:
Importing using the license key file (from account page) -
http://support.brillkids.com/index.php/article/little-reader-importing-your-license-keyImporting using the license key card (inside the box set) -
http://support.brillkids.com/index.php/article/how-to-import-license-key-box-setIf the LR you currently have installed is an older version, simply visit your account page to download the installer for the latest version. The installer wizard automatically detects if you have an existing installation and will proceed with the upgrade process so there is no need to uninstall anything.