Hoping that your data is real XML (including lots of < and > characters which were not visible in your question) you should look at standard deserialization.
This site holds good articles, and has a search facility; which would yield stuff like this: XML Serialization and Deserialization: Part 1
] and XML Serialization and Deserialization: Part 2
If that is not sufficient, have a look at the official documentation, starting here: Introducing XML Serialization | Microsoft Docs
] , and don't skip How to: Deserialize an Object | Microsoft Docs
If it is not real XML, you probably have to dive in the data yourself; to decode a Base64 string, have a look at the Convert Class (System) | Microsoft Docs
] which has a couple of FromBase64... methods.