Mozart's Adagio in 'h' K.deest is a lesser-known work, likely a brief and charming piece that showcases his characteristic melodic grace and clarity.
Focus on clarity of articulation and graceful execution of any ornamentation.
Adagio in h (K.deest) is rated Henle level 5 of 9 — late intermediate repertoire, roughly ABRSM grades 6–7.
You should be comfortable at Henle level 5 — meaning you can already play level 4–5 pieces cleanly. Learning it one level early is possible as a stretch piece, but more than that usually leads to months of frustration.
Adagio in h (K.deest) is available as free public-domain sheet music on IMSLP — the score is linked directly from this page.