watasi ha hurui sumaho wo kaikaemasu.

買い替えます is the polite non-past form of the verb 買い替える, which literally means “to buy and replace (something old with something new).” Key points:

• 買います simply means “to buy.”
• 買い替えます implies you already have an old item and you’re replacing it by purchasing a new one.

In Japanese, い-adjectives like 古い (“old”) directly modify nouns without any additional particle. Structure:

• 古い スマホ = “old smartphone”
  No の is needed because 古い is already an adjective, not a noun.

は is the topic marker. It tells the listener “I’m going to talk about myself.”

• 私 は: “As for me…”
  Using が (私 が) would mark “I” as the grammatical subject, but は is more natural here since you’re making a statement about your own action.

They’re similar but have slight differences:

• 買い替える focuses on replacing an old item with a new one (planned upgrade).
• 買い直す literally means “buy again,” often after returning or because the first purchase was wrong/unsatisfactory.

Japanese verbs often use kanji for the root and kana for the inflection/compounding part:

• 買 (kanji for “buy”)
• い替え (kana + kanji for “replace”)
• ます (polite ending in kana)
  You can write it all in kana (かいかえます), but mixing kanji and kana is standard for readability.