$ pwd
/home/user/project-trunk
$ svn update # (make sure the working copy is up to date)
At revision <N>.
$ svn merge --reintegrate ^/project/branches/branch_1
--- Merging differences between repository URLs into '.':
U foo.c
U bar.c
U .
$ # build, test, verify, ...
$ svn commit -m "Merge branch_1 back into trunk!"
Sending .
Sending foo.c
Sending bar.c
Transmitting file data ..
Committed revision <N+1>.