saihaitatu no zikan wo kaetai toki ha, tuisekibangou ga hituyou desu.

Here, の connects two nouns.

• 再配達 = redelivery
• 時間 = time

So 再配達の時間 means the redelivery time or the time for redelivery.

This is a very common use of の in Japanese: noun + の + noun.

Because 変える means to change, and it takes a direct object.

In this sentence, the thing being changed is 時間, so:

• 時間を変える = to change the time
• 時間を変えたい = to want to change the time

Even though English might naturally say something like change the delivery time or reschedule it, Japanese marks 時間 as the object of 変える.

変えたい comes from the verb 変える.

• dictionary form: 変える = to change
• stem: 変え
• たい added to the stem: 変えたい = want to change

The たい form expresses desire. It is often translated as want to ...

So:

• 変える = change
• 変えたい = want to change

In this sentence, it describes the situation when someone wants to change the time.

とき means time or when.

A clause can come before とき to mean when ...

So:

• 変えたいとき = when you want to change it
• more fully here: when you want to change the redelivery time

The whole part before とき modifies とき.

This is a very common pattern:

• 行くとき = when going / when you go
• 忙しいとき = when busy
• 必要なとき = when needed

The は marks 変えたいとき as the topic or situation being discussed.

So 変えたいときは has the feel of:

• as for when you want to change it...
• if you want to change it...
• when you want to change it...

In notices and instructions, ときは is very common because it sets up a condition or case.

Compare:

• 変えたいとき = when you want to change it
• 変えたいときは、... = when you want to change it, ...

The は gives it a more explanatory, instructional tone.

Because 必要 means necessary, and with 必要, the thing that is necessary is often marked by が.

So:

• 追跡番号が必要です = a tracking number is necessary
• natural English: you need a tracking number

This may feel different from English, because English uses need as a verb, but Japanese is using 必要 as a predicate meaning necessary.

A useful pattern is:

• X が必要です = X is necessary / you need X

It is omitted because Japanese often leaves out subjects when they are obvious from context.

So the sentence does not explicitly say you, but it is understood:

• When you want to change the redelivery time, you need a tracking number.

This is extremely normal in Japanese. In signs, instructions, and customer-service messages, subjects are often left unstated.

Yes. 再配達 is a noun meaning redelivery.

It can also appear in phrases like:

• 再配達を依頼する = request redelivery
• 再配達の受付 = redelivery reception / redelivery request desk
• 再配達の時間 = redelivery time

Japanese often uses noun-heavy expressions in formal writing, notices, and service language, so this phrasing sounds natural.

Yes:

• 再配達 = redelivery
• の = linking particle
• 時間 = time
• を = object marker
• 変えたい = want to change
• ときは = when / if that is the case
• 追跡番号 = tracking number
• が = marker for the thing that is necessary
• 必要です = is necessary

A fairly literal order would be:

As for when you want to change the redelivery time, a tracking number is necessary.

More natural English is:

If you want to change the redelivery time, you need a tracking number.