This coffee is lukewarm/tepid. Could/Can you change it?
This coffee is not hot enough. Can/Could I get another one?
生ぬるいのは、私もイヤです。特に寒い時期はあったかいコーヒーが飲みたいですね。
生ぬるいは、lukewarm あるいは tepid と言います。
This coffee is lukewarm/tepid. Could/Can you change it?
この珈琲は生ぬるいです。別のに変えて頂けますか？
そのほか、「十分熱くない」という意味で、not hot enoughとしても自然です。
This coffee is not hot enough. Can/Could I get another one?
この珈琲は十分熱くないです。別のをもらえますか？
コーヒーは、通常数えられない単語ですが,この場合のようにカフェやレストランでは既にカップに入ったものを想像するので（日本語でも「コーヒー一つ」といいますよね。）
「別の（珈琲）を」は another one でＯＫです。
なお、ご存知とは思いますが、Can~? より、Could~?と聞く方が丁寧です。
ご参考まで。
My coffee is not very hot, may I have a hot cup of coffee, please?
May I another coffee please? This one is lukewarm!
"Coffee" is uncountable, but we are referring to the cup(countable), that is why we ask for "another" or "a".
"Lukewarm", "tepid" it refers to not being warm/hot enough
In English, the context of the situation is often stronger than any words. Body language and tone are also important. Just saying any of these expressions in context should be enough for the attendant to replace your coffee.
A: "I can't drink this, it's cold."
B: "I'll bring you another one madam."
英語では、状況の文脈が、言葉よりもしばしば強いときがあります。ボディラングエッジと声のトーンも重要です。この文脈の中でなら、これらの表現のいずれかで、コーヒーを交換して欲しいと伝えるのに、十分でしょう。
A: "I can't drink this, it's cold."「私これ飲めません。冷たいです。」
B: "I'll bring you another one madam."「もう1杯お持ちしますよ。マダム」
Excuse me, my coffee is cold. Can I get a hot cup of coffee please.
Hi, the coffee is cold, can you bring me another one and please make sure that it is hot.
>Excuse me, my coffee is cold. Can I get a hot cup of coffee please.
*This sentence tells the waiter directly that the coffee is cold and you would like to have a hot one.
,..,..,.,.,.,.,.,.*****,.,.,.,.,.,.,.,.
>Hi, the coffee is cold, can you bring me another one and please make sure that it is hot.
*This sentence shows a but of frustration because the waiter had to know that the coffee should be hot. Now you are telling him that he/she must bring another cup of coffee and make sure that it is hot.
.....................***....................
Excuse me, my coffee is cold. Can I get a hot cup of coffee please.
コーヒーが冷たいので、温かいコーヒーがほしいということを意味する表現です。
Hi, the coffee is cold, can you bring me another one and please make sure that it is hot.
温かいコーヒーを出すべきなので、ウエイターにイライラしていることを意味する表現です。
This coffee is cold, can I have a hot cup of coffee please?
This coffee is not hot enough, may I have another cup please?
You can politely inform the waiter/waitress that the coffee is cold or nor hot enough, and then you can ask for another cup.
Remember to say "please" to be polite and mannerly.
Sorry but the coffee is cold. Could I get another one?
This coffee is cold. I would like to change it for a normal one, please.
To say what you want, you need to use any of the giving sentences. The first one is a more polite version but you could be denied because you seem to nice. The second one is more directive and will work better for you.
日本 
南アフリカ 
カナダ 
イギリス 
アメリカ合衆国 
