If you want to migrate your 3.x website to 5.1, you can use the migration module, which is part of Sitefinity.
The migration module however supports migrating from Sitefinity 3.7 SP4 .NET 4 or .NET 3.5, therefore you might need to upgrade your site to 3.7 SP4 and then proceed with the migration.
For upgrading your 3.x websites, you need a patch, which can be download from your account's downloads page ( please check the attached screenshot ).
Then you can follow the upgrade process:
After you upgrade your project to 3.7 SP4, you can proceed with the actual migration. The our migration guide and blog post can be helpful when it comes to migrating a 3.7 website:
All the best,
the Telerik team
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