简单SVN属性集问题

我对SVN不太了解,也不知道该怎么做。我想更改svn:externals属性的单行。在dir上,svn:externals有几个目录

>svn propget svn:externals dir
a svn://externalproject/a
b svn://externalproject/b
c svn://externalproject/c
d svn://externalproject/d

我想改变,比如, b svn://externalproject/b 到 b/b1 svn://externalproject/b1

that is to say, remove off the externals and replace it with a different one. How do I do that? I'm using svn on the command line, version 1.6.9.