How download 15.1 version

No, you can only update to the latest version of iOS or iPadOS that your device supports. In this case, the latest version is 15.2.1, so if your device supports iOS 15, but you are still on a preceding version of the OS, you can only update to the latest version at the current time.

Is there a reason you specifically need iOS 15.1 rather than 15.2.1?

I’m not able to download the Microsoft word and update other apps

? I’m using and getting updates for Word etc. on iPadOS 15.2.1…

Microsoft supports iOS/iPadOS 14 and higher. This includes iPadOS 15.2.1.