The password for the next level is stored in the file data.txt, where all lowercase (a-z) and uppercase (A-Z) letters have been rotated by 13 positions
Okay lets see what the wikipage has to say about Rot13, it probably is something like the caesar algorithm.
I found this on the wiki as an example: tr ‘A-Za-z’ ‘N-ZA-Mn-za-m’, so lets try this with the data.txt. We can cat the file and then pipe it to the tr command.
cat data.txt | tr 'A-Za-z' 'N-ZA-Mn-za-m'
The tr part basiscly changes everything from letter capital A to z (so a-z and A-Z) by 13 places (N-ZA-M)
there is the password: 5Te8Y4drgCRfCx8ugdwuEX8KFC6k2EUu