I imported my already working project on another computer and it started to download dependencies.
Apparently my internet connection crashed and now I get the foll
-U seems to force update of all dependencies.
If you want to update a single dependency without clean or -U you could just remove it from your local repo and then build.
The example below if for updating slf4j-api 1.7.1-SNAPSHOT:
rm -rf ~/.m2/repository/org/slf4j/slf4j-api/1.7.1-SNAPSHOT
mvn compile