How to change mySQL tmp directory

[raed]

Hi,

I am new to work under unix and linux, the administrator left us stuck with an application running with mySQL 4.1 on a unix based server
any query that includes big data cause errcode 28, which seems due to the size of the \tmp directory of mySQL

my question:

how can i know where did the admin locate the mySQL tmp directory
and how can i change it to bigger one.

any help is appreciated, the application is live, and every day costs us loss of clients. Please be detailed in your replies ..we nearly have no experience with unix and linux environment

Thanks

[Skevin]

tmpdir = /path/to/mysqltempdirectory


Solomon Chang