Click or open File in the top menu bar, then Exit to exit the program and return to Windows ™. A prompt appears if you have a completed calculation that you have not saved, with a chance to save it.
Although highly experienced software developers use their best efforts to create stable programs, with literally millions of code instructions present in modern sophisticated computer programs, plus the added complication of hundreds of 'platform' and computer operating system combinations, configurations etc. with which the programs must be designed to operate, it cannot be guaranteed that none will be completely immune to 'crashes'. A detailed description of the keystrokes and operations preceding any failure, as well as the system failure reference numbers following the event, will help developers track down and perhaps eradicate the problem.