Ok I figured out by myself through this forum soon after I make the post lol
For any beginner who is wondering, here is what I have:
myDir = uigetdir
myFiles = dir(fullfile(myDir,'*.tiff'));
outputDirectory = 'C:\Users\Desktop\your folder';
mkdir(outputDirectory);
for k = 1:length(myFiles)
    baseFileName = myFiles(k).name;
    fullFileName = fullfile(myDir, baseFileName);
    imageData = imread(fullFileName);
    pngFileName =sprintf('pngFrame_%03d.png', k);
    pngFilePath = fullfile(outputDirectory, pngFileName);
    imwrite(imageData, pngFilePath);
end
For the experts, please let me know if there is a more efficient way to do it. Thank you so much!
