Can I Upgrade SQL Server 2008 R2 Express to SQL Server 2014 Developer edition
No you cannot do inplace upgrade from SQL server 2008 R2 express to SQL server 2014 developer edition. Its not supported as per Version and edition upgrade supported matrix 

You can only upgrade to below using inplace upgrade method


  SQL Server 2014 Enterprise
  
  SQL Server 2014 Business Intelligence
  
  SQL Server 2014 Standard
  
  SQL Server 2014 Web
  
  SQL Server 2014 Express


But there is other option to do side by side migration. You install SQL server 2014 DEV edition. You backup your user database from SQl server express and restore it on SQl Server 2014.

You can migrate logins using This support article