Given the pattern, this might be (A↔Z, B↔Y, etc.). Let's test first word:
To decode, one can use frequency analysis: in English, common letters like E, T, A appear often. Comparing the ciphertext's letter frequencies with standard English frequencies helps guess the shift. Download- shrmwtt tjyb shyqha ydklha ksha wkhrm ...
Better approach: Look at the whole string as possibly "Download" being the first word in plaintext. If "shrmwtt" = "Download" , let’s check first letter: D (4) → s (19) means shift +15. Given the pattern, this might be (A↔Z, B↔Y, etc