while you're probably not going to implement this in simulink but take a look at https://www.mathworks.com/help/vision/ref/chromaresampling.html which covers how it should be done for 4:4:4 to 4:2:2.
Not sure what the standard practice is for the down sampling as to which of say the 2x4 pixel subsets are used or average together to genearte the block but overall you're combining/binning the colors 2x2.
edit: also take a look at https://www.mathworks.com/matlabcentral/fileexchange/36417-yuv-files-reading-and-converting which is for YUV (yes i know you're looking for YCBCr)
where
yuv(:,:,1) = double(Y);
yuv(:,:,2) = imresize(double(U),2,'bicubic');
yuv(:,:,3) = imresize(double(V),2,'bicubic');
you can see that layer 2 and 3 (U and V) are resampled/resized to match the 4:2:0 scheme.
the file exchange that was mentioned in other answer posts appears to have been removed https://www.mathworks.com/matlabcentral/answers/65149-converting-ycbcr-4-4-4-to-4-2-0?s_tid=gn_loc_drop
