Module doesn't appear on update.php
Some things to check:


Is your update function placed in [modulename].module or in [modulename].install? It should be in the install file.
Is the function really called hook_update_6031? in that case, replace 'hook' with the name of your module. For instance, an update function for the comment module could be called comment_update_6003.


If the above does not help, please update your question with the code of your update function. Also check the value in the schema_version column in the system table for your module - for the [modulename]_update_6031 function, it must not be less than 6031, but not -1.