MODIFY table

Hey guys, i have this code in my template file, what would I put in my PHP file to make what ever I change in the textbox, changes the feild in mysql.

The table is extra_fields >> gamename.

<td>In-game nick:</td>    <td><input type="text" name="gamename" id="gamename" value="{$i.gamename}"/>

UPDATE extra_fields
SET gamename = $gamename
WHERE …