You can uninstall Sitefinity through your Control Panel.
Then if then you're still having problems installing Sitefinity 5.3.3900 via the SitefinityTrialSetup you can try downloading "Project manager" zip file from here -> then unzip it and start the Sitefinity.exe.
If you're still having some troubles, feel free to contact us again.
Do you want to have your say in the Sitefinity development roadmap? Do you want to know when a feature you requested is added or when a bug fixed? Explore the Telerik Sitefinity CMS Ideas&Feedback Portal
and vote to affect the priority of the items