This depends on the phone used. The latest Nokia phones are able to keep personal data in place - but there is an issue with older phones. It is highly recommended to take a backup before the firmware update.
If the applications were downloaded from Nokia Store you can easily redownload them by signing into Nokia Store website and accessing My Stuff.
This topic is valid for:
Nokia C6,Nokia E73,Nokia E7,Nokia E6,Nokia X2-01,Nokia 808 PureView,Nokia 700,Nokia 5310