You can switch between Debug and Release build configurations by selecting the project you want to change the build configuration of in the Project Explorer view, then using one of the below methods:
Select the menu item Project->Build Configuration->Set Active and select Release or Debug as necessary
Use the drop down arrow next to the 'sundial' (Manage configurations for the current project) icon on the main toolbar (next to the 'hammer' icon) and select Release or Debug as necessary. Alternatively you can use the drop down next to the 'hammer' icon to change the current configuration and then immediately trigger a build.
Right click in the Project Explorer view to display the context sensitive menu and select Build Configurations->Set Active entry.
Changing the build configuration of multiple projects
It is also possible to set the build configuration of multiple projects at once.
To do this, you first of all you need to select the projects that you wish to change the build configuration for in the Project Explorer view - by clicking to select the first project, then use shift-click or control-click to select additional projects as appropriate. If you want to change all projects, then you can simply use Ctrl-A to select all of them.
Note it is important that when you select multiple projects, you should ensure that none of the selected projects are opened out - in other words, when you selected the projects, you must not have been able to see any of the files or the directory structure within them. If you do not do this, then some methods for changing the build configuration will not be available.
Once the required projects are selected, you then need to simply change the build configuration as you would do for a single project.