You can change these parameters using a php.ini (or php4.ini if you are using PHP4) file. Simply create a file of this name in the same directory of your script (or update the existing file), with the content:-
upload_max_filesize = 20M ; post_max_size = 20M ; max_execution_time = 60 ;
(this example would increase the maximum upload to 20MB, and timeout to 60seconds).