how to remove ffmpeg ? and install again
Removal

If you installed with HomeBrew, the following Terminal.app command will perform the uninstall:

brew uninstall ffmpeg


If you are not sure how you installed ffmpeg, try running the command alone. The output will include a line containing the binary's location on your Mac:

ffmpeg


Look for output similar to:

ffmpeg version 1.2.4 Copyright (c) 2000-2013 the FFmpeg developers
  built on Nov 21 2013 11:36:01 with Apple LLVM version 5.0 (clang-500.2.79) (based on LLVM 3.3svn)
  configuration: --prefix=/usr/local/Cellar/ffmpeg/1.2.4 …


The key information in this extract is /usr/local/Cellar/ffmpeg/; this provides the location and that ffmpeg was installed as a HomeBrew Cellar.

If you do not see this output or it does not mention Cellar, please expand your question with the output you do see.

Alternative Installations

It is critical to know how you installed ffmpeg in order to know how to remove it. If may just be removing a folder, or it may be hunting around in /usr/local/ and /usr/bin folders for installed binaries.

Easy removal and upgrading is a strong argument for using a package manager for command line tools. Well supported package managers on the Mac include brew and MacPorts.

Re-installation

To install ffmpeg again, after removing the prior version, use brew to handle the entire process with:

brew install ffmpeg