Converting Celsius and Fahrenheit in C++
You might as well break the habit of using using namespace std. It involves a little more additional code, but it'll be helpful in the future, especially when using header files.
You don't need those global variables outside of main(). You should really avoid global variables unless absolutely necessary since they can be changed from any scope and you may not even know it. Just put those two variables inside if main() before their use.
farenheit is spelled incorrectly; it should be fahrenheit.
What if the user uses an uppercase 'C' or 'F' instead? You should account for those as well:

case 'c':
     'C': {




case 'f':
     'F': {

It may be more readable to multiply by both 9.0/5.0 and 5.0/9.0 respectively instead of 1.8 since that's most common with these conversion formulas. Make sure not to use integers for this, otherwise the division will truncate towards 0 (in this case, you'll end up with 1 instead of 1.8 and 0 instead of 0.555556).
You may find it better to use double instead of float in most cases and because the former has more precision.
For the next step up, I would recommend adding additional functions since main() shouldn't have to do all of the work.

You can simply acquire value from main(), pass it to the applicable function, then return the result and display it. It'll also allow you to avoid duplicating code in the switch, such as having multiple inputs for value, which is very nice and will be essential in every other program you create. I'll let you come up with that yourself.
If you want to make your program more useful, you can also add Kelvins.
You should probably put that tagline cout in a comment instead, otherwise it may be annoying to see it come up each time when only a result is expected to be printed.
You don't really need return 0 at the end of main() since success is already guaranteed at that point, though it won't hurt anything either.