How to reinstall ubuntu 18.04 on same instance?

I would suggest you ...

1. Create a new instance
2. Transfer the data from the old instance to the new one.
3. Detach the static IP from the old instanced
4. Attach the static IP to the new instance

david

Thanks for reply & help david.

I did not create static ip for the instance before. I was just using the given public ip.
any way to retain that now or i should just go for the new instance?

You won't be able to take the instances current IP on a new instance unless it's static.

You might want to try assigning a static IP to the current instance and MAYBE you'll get the current IP assigned as static (I'm not sure if that works).

Otherwise, I would suggest just creating a new instance, transfer your data, and reconfigure the OS.