I have checked the dictionary, the following are the meanings for Idiom and its components. could not get how ‘keep conventional ways’ was the meaning derived. can share more light on this? thanks
按部就班 ànbùjiùbān follow the prescribed order; keep to conventional ways
安 ān calm, quiet, peaceful, tranquil 按 àn put hand on, press down with hand 部 bù part, section, ministry, department
按部就班 is not wrong and is in fact etymologically more correct:
選義按部，考辭就班 （陸機《文賦》）
To obtain choice ideas in close observation of things in categories, and elect expressions that will fall in happy order (trans. Chen Shixiang 1951)
This was an advice by Lu Jin from the Jin dynasty on how essays should be written. Of course in modern Chinese the idiom has evolved to describe things other than writing.
Is 按步就班 wrong? Etymologically, definitely yes, but it is understandable, and there are cases where these variants that appease the modern reader become acceptable. Consider the etymologically correct 名副其實 （曹操《與王修書》） and its variant 名符其實. Personally, I would stick to 按部就班 and 名副其實. You should especially do so if you are taking a language exam. I would also interpret 部 and 班 in classical Chinese – good practice if you are learning idioms.
Also, try not to interpret 按 as 'to press'. It is similar to meaning 按照 ('according to') here.
按部就班 also 按步就班
按 = 按照 = following; according to
部 = 部門 = sections; categories
步 = 步骤 = procedure; steps
就 = complete
班 = shift; duty; work
按部就班: according to sections/ categories to complete works (do things in ways according to each different section/category required)
按步就班: following steps to complete works (do things step by step)
Both versions imply 'do things accordingly' = 'keep conventional ways'
