1) how do I merge this two?
I didn't quite understand what you mean by merge XML and XSD files. XSD are schema files and XML file can be validated against this schema.
2) how do I read the xml from my C code?
3) how do I modify this file after the central device (BLE) sent information?
Cypress doesn't have any library to read the XML, but you can make use of any third party C libraries like Libxml2 library to parse the XML files.
4) is there an example?
No, do not have any example.