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She decided to drink water instead of soft drinks to lose weight.
Literal
She [topic-は] body-weight [object-を] reduce [in-order-to-ために] soft-drink [genitive-の] instead-of water [object-を] drink [decided-ことにした].
Three patterns layered into one sentence. ~ために gives the goal ('in order to lose weight'). ~の代わりに ('instead of, in place of') marks the substitution ('water instead of soft drinks'). ~ことにする expresses a personal decision ('decided to drink water'). Compare ~ことになる ('it has been decided'), which is more passive — outcome-of-circumstances. ソフトドリンク is a katakana loanword from English. 体重を減らす ('reduce body weight') is the standard collocation for 'lose weight,' with 体重 as the object reduced.