If you have been relying on the Release Management 2013 Client to push your builds into Release Management and you have uninstalled the 2013 release and installed the 2015 release you will have to edit the XAML file and change the 12.0 references to 14.0. Here is the post from Microsoft with the Build Process Template for 2013.
In this particular environment I have a TFS 2015 Update 1 RC1 server installed and using Visual Studio Enterprise 2015 and Release Management Client 2015. Locate the Build Process Template used by your Build Definition and open it using notepad or your favorite text editor and locate the references to ReleaseManagement\12.0\Client and change them to ReleaseManagement\14.0\Client.
Don’t forget to check-in the changes of your Build Process Template into version Control and then trigger a new build to test your changes. Here is an example of the build running successfully and showing up in Release Management 2015.
If you are looking for information on the next version of build, here is a link to Team Foundation Build 2015 which I think you’ll find much easier to adopt for your build processes.