Tjss Ly Lhm Mharm Mn Tht Qb A...: Download- Fydyw

"Download- fydyw tjss ly lhm mharm mn tht qb a..."

Given this is a puzzle, and you asked for a "write-up", I'd conclude the intended decoding is a (or 21 forward), yielding: Download- fydyw tjss ly lhm mharm mn tht qb a...

Shift backward by 1 didn't work. Maybe shift forward? "Download- fydyw tjss ly lhm mharm mn tht qb a

Try could : c(3), o(15), u(21), l(12), d(4). Differences: c→f=+3, o→y=+10, u→d=-17, l→y=+13, d→w=+19 — no. and you asked for a "write-up"

Let's brute force Caesar mentally:

But maybe it's a Caesar shift of 21 (or -5)? Let's check fydyw shift back 5: f(6)-5=1→a, y(25)-5=20→t, d(4)-5=25→y, y→t, w(23)-5=18→r → atytr — no.