First, we need to make sure we have all our changes in our working copy checked in before we proceed. Then we check out the trunk and switch to it:
svn co http://server/repository/project/trunk
svn switch http://server/repository/project/trunk
Now we can merge:
svn merge http://server/repository/project/trunk http://server/repository/project/branches/MyBranch
Finally we can commit to the server:
svn ci -m "Merge MyBranch to Trunk"
Finished. :)