To update a single subscription select it in the feed list and select "Update" or "Update Folder" from the program menu or the context menu from within the feed list.
To update all subscriptions at once use the "Refresh" button from the toolbar or the option "Update All" from the "Subscriptions" menu.
To automatically update all subscriptions you can set a global update interval with the resolution of one minute. To do so you have to use the preferences dialog. To learn more about this refer to the Program Preferences section.
If you like to have some subscriptions to be updated more or less frequently than the global default update interval you can set an per-subscription update interval in the subscription properties. To do so select a subscription from the feed list and select "Properties" from the "Feed" menu or the contextual menu. Note: you should not update a feed more often then the content is changing. Think of the web traffic some thousands of clients polling each minute could create!
To help with the decision for a good update interval some subscriptions provide an own update interval. When you create a new subscription this value will be set as the default update interval. This setting will overrule the global default update interval.
On startup Liferea automatically updates all out-dated subscriptions. If you don't like this you can disable this behaviour in the program preferences. To learn more about this refer to the Program Preferences section.