base directory of your WordPress installation is not writable by the webserver.

i want to upgrade my wordpress to 2.9.2
so i am using instant upgrade plugin using http access

and i am getting following error msg
“”””HTTP permissions check

HTTP permissions check failed.
Not all required files/directories are writable.

On most servers, your WordPress files are not writable by the webserver by default. In that case, it is strongly recommended that you use the FTP method, if you have FTP access to your WordPress installation. If you don’t want to use the FTP method for whatever reason, you must make the a number of files and directories writable by the webserver.

* Ok: InstantUpgrade’s working directory is writable by the webserver.
* Error: The base directory of your WordPress installation is not writable by the webserver.
* Error: One or more of your WordPress files are not writable by the webserver.”””””

pls help me how to update my wordpress