Those that have access level 7, can be modified by ini_set() function. And some can be set using .htaccess file. And some need to have admin level access.
If you want to use .htaccess file then you need to paste the following code into you .htaccess file.
php_value upload_max_filesize 100M php_value post_max_size 105M php_value max_execuetion_time 300 php_value memory_limit 64M
you need to set post_max_size to larger size than upload_max_filesize, as there could be multiple upload field in a form.
Now, while you are using .htaccess file, the server may not support the directive changing command that you are trying to modify. I mean your server need to have AllowOverride property On for the specific directive that you are trying to modify. you need to contact with your hosting server support.
There is another way, you can create a php.ini file with the configuration directive that you want to change. For image uploading in wordpress, you can create the following php.ini file.
;;;;;;;;;;;;;;;; ; File Uploads ; ;;;;;;;;;;;;;;;; upload_max_filesize = 100M; post_max_size = 105M; max_execution_time = 300; ;End:
you need to place it under the wp-admin folder.

