Assalamualaikum sir.

I want to ask you a question. My question is - in the 10th verse of sura az zumar Allah says : "Say, o My servants who believe......." , why Allah tells Muhammad (s) to address us as servant? We are servant only of the Almighty Allah. Please sir, answer me.

You misread.
Allah is calling you, his servants, and Allah is addressing or calling you (his believers) his servants. That is how he tells Muhammad, who to call (tell)

The "my servants" is the title or name for the servants of Allah.

Hello Khaledh33,

I agree with FR_Chuck that you misread it, please open the link below it may give you some more information, Thanks.

https://www.al-islam.org/faith-and-reason-ayatullah-mahdi-hadavi-tehrani/question-5-being-allah-s-servant