delete subfolder\filename.csv %relative path to current directory
But you shouldn't rely on the main directory and instead use full paths everywhere in your script. This way the data can be located anywhere independently on where the code is.
datafolder = 'C:\somewhere\somefolder'
%importing some data with whichever function you normally use. Using readmatrix here for demo:
filename = 'somefile.csv';
data = readmatrix(fullfile(datafolder, filename));
%deleting a particular file in the data folder:
delete(fullfile(datafolder, 'somefile.txt'));
%saving an image
imwrite(img, fullfile(datafolder, 'fancyimage.png'))
%etc.