.repo/repo/repo sync 更新代码后

.repo/repo/repo sync
Fetching project rk/kernel-stable
Fetching project rk/kernel
error: Cannot fetch rk/kernel-stable (GitError: --force-sync not enabled; cannot overwrite a local work tree. If you're comfortable with the possibility of losing the work tree's git metadata, use `repo sync --force-sync kernel` to proceed.)
Exception in thread Thread-1:
Traceback (most recent call last):
  File "/usr/lib/python2.7/threading.py", line 810, in __bootstrap_inner
    self.run()
  File "/usr/lib/python2.7/threading.py", line 763, in run
    self.__target(*self.__args, **self.__kwargs)
  File "/home/xxx/rk3399pro/.repo/repo/subcmds/sync.py", line 270, in _FetchProjectList
    success = self._FetchHelper(opt, project, *args, **kwargs)
  File "/home/xxx/rk3399pro/.repo/repo/subcmds/sync.py", line 314, in _FetchHelper
    prune=opt.prune)
  File "/home/xxx/rk3399pro/.repo/repo/project.py", line 1229, in Sync_NetworkHalf
    self._InitGitDir(force_sync=force_sync)
  File "/home/xxx/rk3399pro/.repo/repo/project.py", line 2267, in _InitGitDir
    raise e
GitError: --force-sync not enabled; cannot overwrite a local work tree. If you're comfortable with the possibility of losing the work tree's git metadata, use `repo sync --force-sync kernel` to proceed.
Fetching project rk/prebuilts/gcc-linaro-6.3.1-2017.05-x86_64_aarch64-linux-gnu
Fetching projects:   1% (1/57)  Fetching project rk/prebuilts/gcc-linaro-6.3.1-2017.05-x86_64_aarch64-linux-gnu
Fetching projects:   5% (3/57)  
error: Exited sync due to fetch errors
请问是什么原因如何解决好

你本地内核代码有改动，错误写的这么清楚了，也告诉你要怎么做了。
If you're comfortable with the possibility of losing the work tree's git metadata, use `repo sync --force-sync kernel` to proceed.