MATLAB is an application and this productive software is used for environment for engineers and scientists. You have to control the application with code and without using code, you will not get many important features. For extracting movie frames or separating frames, you have to use code. With the application, you will not get all tool or function for controlling the MATLAB easily, you have to use code. You need to visit the YouTube link https://www.youtube.com/watch?v=AI-1ch6CHkI and learn how to use code in the proper section. You can use below code on the proper section for separating frame or frames. You can copy-paste the below code to the proper section.
for img = 1:a.NumberOfFrames; filename=strcat(‘frame’,num2str(img),’.jpg’);
b = read(a, img); imwrite(b,filename);
Once you have done, you should save the file or click on play button and save. On the left side, you will get all frames as images. You see all images and remove if you like. Once you have removed the file, you cannot get back easily. So, you should careful when follow the process. You can complete the steps or remove frame in more critical ways but I think that this process is enough for solving the problem what you have faced.