Like は and も, the possessive particle の can also append itself to other particles to help modify nouns.

友達ともだちへ 手紙てがみを おくります。
To send a letter to a friend.
友達ともだちの 手紙てがみ
A friend’s letter.
友達ともだちへの 手紙てがみ
A letter to a friend.

Note that にの is not a valid combination. We must use の if we want to combine the directional particle with へ!

友達ともだちから 手紙てがみを もらいます。
To receive a letter from a friend.
友達ともだちからの 手紙てがみ
A letter from a friend.
先生せんせいと 会話かいわを します。
To have a conversation with a teacher.
先生せんせいとの 会話かいわ
A conversation with a teacher.
電車でんしゃで 通勤つうきんします。
To commute by train.
電車でんしゃでの 通勤つうきん
A commute by train.
東京とうきょうから 京都きょうとまでの 距離きょりは およそ380キロメートルです。
The distance from Tokyo to Kyoto is about 380 kilometres.

In writing you may occasionally see へと used in combination but this does not change the meaning or grammatical usage. This will not come on the N5 or N4, but you may see it later.

日本にほんは、20世紀にじゅうせいきに 産業社会さんぎょうしゃかいから 技術社会ぎじゅつしゃかいへと かわりました。
During the 20th century Japan moved from an industrial society to a technological one.
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