by now, you cannot use it from inside SNAP (see here: Error: sen2cor and here Sen2Cor 2.4.0, Stand-Alone Installers, How To Install)
But you can start it from the command line.
Simply open the command prompt in a folder where your S2 data (the folder ending with .SAFE) is located (https://www.techsupportalert.com/content/how-open-windows-command-prompt-any-folder.htm).
There you type:
L2A_process --resolution 10 NameOfTheFolder.SAFE
Replace NameOfTheFolder.SAFE with the actual folder name. Instead of typing it down, you can enter the first letters and then hit the tab key to complete the full path name.
All this is described in the manual, chapter 3.3.1
To be able to run it from any location, you need to add the path of the folder where sen2cor is installed to your system’s PATH variable: https://www.howtogeek.com/118594/how-to-edit-your-system-path-for-easy-command-line-access