Mozart's K. 5b is a charming and relatively simple early piano piece. It embodies the lightness and grace typical of his juvenile compositions.
Focus on clean articulation and a light touch, especially in the Alberti bass accompaniment.
Andante in B-flat major (K.5b) is rated Henle level 2 of 9 — beginner repertoire, roughly ABRSM grades 2–3. In the RCM system it corresponds to about level 3.
You should be comfortable at Henle level 2 — meaning you can already play level 1–2 pieces cleanly. Learning it one level early is possible as a stretch piece, but more than that usually leads to months of frustration.
A typical performance of Andante in B-flat major (K.5b) lasts about 0.8 minutes.
Andante in B-flat major (K.5b) is available as free public-domain sheet music on IMSLP — the score is linked directly from this page.