Is there an expression or idiom which corresponds to changing one's mind very often?

Ironically, what motivated my question was a French-language textbook. In French, there is an expression "changer d'avis comme de chemise" which would translate to "changing one's mind as often as one's shirt". Is there an equivalent expression or idiom in English?

There are similar expressions. But many are likely to be regional or to come into and then fall out of fashion.

He changes with (or as often as) the weather.

Fickle as the sea. —William Cullen Bryant

Words related to changeable, from https://www.merriam-webster.com/thesaurus/changing:

capricious, changeable, changeful, choppy, fickle, fluid, inconsistent, inconstant, mercurial, mutable, uncertain, unsettled, variable, volatile

The adjective phrase "fickle-minded" means "inconstant", 'prone to frequent changes" (ref.). The expression "to be fickle-minded" is therefore one possibility.