folder = 'Path\to\your\directory';
old_ext = '.txt'; % original extension
new_ext = '.dat'; % new extension
old_files = dir(fullfile(folder,['*' old_ext])); % files with extension old_ext in folder
old_names = fullfile(folder,{old_files.name}); % full-path names of those files
new_names = regexprep(old_names,[old_ext '$'],new_ext); % new file names with extension new_ext
