To download and debug your program, you should just select your project, then using the context menu (right click) select “Debug As >>> Ac6 STM32 C/C++ Application” (also available from the Run menu) and it should automatically download your program, start it (with a breakpoint set at main) and switch to the Debug perspective, where you will be able to single step your code, set or remove breakpoints, look and modify variables...
To come back to the normal C/C++ perspective, just click on the “C/C++” button on top right of the window.
By the way, dit you read the various documentation pages on the web site? If not I advise you to read and practice the Getting started with System Workbench for STM32 guide as well as all the pages under System Workbench for STM32
Thanks for your patience,