First of all make sure your subscription support high-speed data transfer such as GPRS, EDGE or 3G. If you're uncertain about this, contact your operator.
Second, make sure you have correct Internet settings in your phone. These can be obtained from this site.
Please note that an update is only possible when a new release of your phone's software is available. The time when a new release of the software is available may differ between operators.
Some operators have a size limit for downloads. If the download package needed to update your phone exceeds this limit you may get a massage similar to 'Resume data transfer'. Just press 'Yes' to continue download.

