This can be done by updating the setting in your site.ini files. You can update the file /settings/override/site.ini.append.php or just for a specific siteaccess trough the file /settings/siteaccess/*site_access_name*/site.ini.append.php.

Your settings should be like this:

[RSSSettings]
# Cache Time in Seconds
# 
# The timespan a RSS feed is served from the cache in seconds.
# After the end of the timespan, the RSS feed is regenerated and
# again served from the cache.
#
# This means that changes to the content being fed show up
# after the maximum cache time at the latest.
#
# "CacheTime = 0" turns off cacheing.
CacheTime=0

It's important to notice that reducing the cache time to 0 seconds can affect the over all performance of your server.