Assuming you're already using the m2eclipse plugin, if you right-click on a Maven project there is a Maven submenu.
Two of the actions available in this submenu are:
I've found an answer that says that "Update snapshots" is equivalent to using the -U parameter for mvn.
If "update snapshots" is the same as mvn -U what does "update dependencies" do?
It largely depends on how snapshot updating policy is set in your environment. By default Maven is supposed to update snapshots once a day (hence the flag -U and "Update Snapshots" action to force the snapshot update), but this policy can be overwritten locally or at repository manager level.
The "Update Dependencies" does pretty much the same as "Update Snapshots" action, except forcing updating snapshot artifacts. Both actions reset in-memory project model created by M2E to keep track of dependencies, including those that came from Eclipse Workspace.