How can I make this C++ code to obtain passwords more efficient?
Regarding your program's efficiency, remember that premature optimisation is the root of all evil. Looking at your program, I can tell that your program will be IO bound - meaning that the computer will spend longer waiting for the user to input the data than actually executing your program. Moreover, functions like std::cout which deal with output, include inevitable slow system calls, which will be far costly in time than your algorithm. So performance should not be a concern here ! :)
The only thing I would say about your program efficiency is const string PASSWORD = &quot;PASSWORD&quot;; which will use unnecessary copying and heap allocation. It is better to preserve the original type of the string like so:
const auto PASSWORD = &quot;PASSWORD&quot;;  (Here auto (assuming c++11) deduces the type for you, but the real type of the string is const char* if you want to know)
CODE REVIEW
These are the style/ readability points I would make on your code.

Use proper indentation (this might due to copy pasting on codereview so I don't blame you for that)
Do not use using namespace std - this will lead to naming collisions and all sort of horrible errors. Better to use the std:: every-time instead.
You might consider to put the variable PASSWORD inside main as a static const variable, this will limit the visibility of PASSWORD.
Use a better variable name than c.
If your program gets bigger, consider wrapping your algorithm inside a function, this might help to make the main function a bit neater.

That's all! Well done, I would say generally this is very well written, readable c++ code.