Acetic acid is a weaker acid than Formic acid. This can be explained using

Acetic acid has methyl group attached to carboxylic acid COOH that shows positive inductive effect and increases the electron density on the C of COOH group, thereby reduces its acidity. While in formic acid i.e. HCOOH no such electron donating group is present hence will have higher bond polarity of C−O bond and shows greater acidity than acetic acid.