Acetic acid (CH3COOH) convert to acetaldehyde(CH3CHO)
Firstly you convert -OH group in acetic acid to Cl and use PCl5 for this reaction.
CH3COOH+PCl5 to gives CH3COCl and after these use H2/Pd in present of BaSO4 to convert Cl group in CH3COCl to CH3CHO.
Regards