How to write it？
リンゴ
アップル
りんご
林檎
When writing apple in Japanese, You can use either Katakana, hiragana or kanji.
You can also use the English-like sound アップル in Katakana.
Advanced Tips
We usually use リンゴ or りんご to discrive an apple itself.
E.G.
私はりんごが好きですor 私はリンゴが好きです
/ I like an apple.
We don’t use アップル for an Apple itself. we use アップル for something mede of apples including an apple pie.
E.G.
昨日、アップルパイを作りました
/ I made an apple pie yesterday.
you can see 林檎 in novels or poems. the Image of 林檎 is a bit stiff and sounds clever. So, sometimes, junior high school students who try to look like adults want to use the word.
E.G.
彼女は林檎の花のように可憐だった
/ She was as pretty as an apple blossom.
I also want to give you some information. you may see あっぷる.
I think it’s rare, but we use this when we want to express a person who isn’t used to pronouncing it. For example, in a scene where a little child manages to say it for the first time.
E.G.
姪の花子はアップルパイを見て「あっぷる」と言いました
/ My niece Hanako said “ an apple” when she saw an apple pie.
Kotoba no Hibiki 