hurui eakon wo atarasii eakon ni kaetai desu.

In Japanese, modifiers like adjectives come before the noun they describe.

So:

• 古いエアコン = old air conditioner
• 新しいエアコン = new air conditioner

This is the normal pattern in Japanese, unlike English, where learners sometimes expect more word rearranging. Here it works very similarly to English.

They show the roles of the two nouns:

• 古いエアコンを = the old air conditioner is the thing being changed/replaced
• 新しいエアコンに = the new air conditioner is what it is being changed to

So the pattern is:

A を B に 替える
= replace A with B / change A into B

In this sentence:

• A = 古いエアコン
• B = 新しいエアコン

The particle に often marks the result, destination, or target state of a change.

Here, the old air conditioner is being changed into or for a new one. So に marks the result of the replacement.

A useful way to think of it is:

• X を Y にする = make X into Y
• X を Y に替える = replace X with Y

So に does not mean exactly the English preposition to in every case, but here it marks what the old thing is being replaced with.

替える is the dictionary form, meaning to replace.

To say want to replace, Japanese adds たい to the verb stem:

• 替える → stem 替え
• 替え + たい = 替えたい

So:

• 替える = replace
• 替えたい = want to replace

This is a very common pattern:

• 食べる → 食べたい = want to eat
• 行く → 行きたい = want to go
• 替える → 替えたい = want to replace

Because ～たい behaves grammatically a lot like an i-adjective.

So:

• casual: 替えたい
• polite: 替えたいです

This is why you do not say 替えますたい. The たい form already changes the verb, and then です makes the whole sentence polite.

Compare:

• 行きたい。 = I want to go.
• 行きたいです。 = I want to go. (polite)

Japanese often leaves out the topic or subject when it is obvious from context.

So even though English would usually say I want to replace..., Japanese often just says:

古いエアコンを新しいエアコンに替えたいです。

The understood subject is usually I.

If you wanted to make the topic explicit, you could say:

私は古いエアコンを新しいエアコンに替えたいです。

But in many everyday situations, 私は is unnecessary.

This is a very common question, because both can be read かえる.

A simple distinction is:

• 替える = replace one thing with another
• 変える = change/alter something

In this sentence, 替える is used because you are replacing an old air conditioner with a new one.

Examples:

• 電池を新しいものに替える = replace the batteries with new ones
• 予定を変える = change the schedule

In real life, Japanese has several かえる verbs with different kanji, such as 替える, 変える, 換える, and 代える, but for learners, the key point here is that 替える fits the idea of replacement.

Yes, Japanese often repeats nouns when it helps make the sentence clear.

So this sentence is perfectly understandable as written:

古いエアコンを新しいエアコンに替えたいです。

But in natural conversation, people might shorten it if the noun is obvious:

• 古いのを新しいのに替えたいです。
  = I want to replace the old one with a new one.

Or:

• エアコンを新しいのに替えたいです。
  = I want to replace the air conditioner with a new one.

So the repetition is not wrong at all; it is just a bit more explicit.

Yes, it is natural and correct.

It clearly expresses the idea of wanting to replace an old air conditioner with a new one.

Depending on context, a native speaker might also say:

• 古いエアコンを新しいものに替えたいです。
• エアコンを新しいのに替えたいです。
• 古いエアコンを交換したいです。
  if the context already makes the replacement idea clear

But your original sentence is a solid, natural sentence.

エアコン is a loanword from English air conditioner.

A few useful things to notice:

• It is written in katakana, which is common for loanwords.
• Japanese shortens many borrowed words, and エアコン is a shortened form.
• It usually refers to an air-conditioning unit, and depending on context it may imply both cooling and heating functions.

So even though it comes from English, it behaves like a normal Japanese noun.

Usually it is fine, especially when you are simply stating what you want to do.

• 古いエアコンを新しいエアコンに替えたいです。
  = I want to replace the old air conditioner with a new one.

This is a straightforward statement of desire.

However, if you are talking to someone who has the power to approve or deny it, softer expressions may sound more polite, for example:

• 古いエアコンを新しいエアコンに替えたいと思っています。
  = I’m thinking of replacing the old air conditioner with a new one.
• 古いエアコンを新しいエアコンに替えたいのですが。
  = I’d like to replace the old air conditioner with a new one, but...

So 替えたいです is correct and natural, but Japanese often uses softer phrasing when making requests or discussing plans with other people.