It sounds like you're not even sure how to approach the problem. Here's some pseudocode that may be helpful:

for each line in file:
  extract (old_name, new_name) from line
  new_name_of{old_name} = new_name

for each file in directory:
  new_name = new_name_of{file}
  rename file, new_name
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As toolic mentioned, you haven't provided us quite enough information to be completely helpful. In other words, this pseudocode may not be exactly what you need, so caveat emptor.

Additionally, do you need to do this for one directory, or a whole bunch of them? If you need more than one, you'll have to build an additional looping structure. Even for one, you should probably make a function to do the work for you. Then you can call the function in a loop, if you need to.