Need more info:
Which dropdown control is the app using? 3rd party or native Android?
Is the dropdown being filled by a web service or static?
Did you try to record and see what got recorded?
Did you instrument the apk file?
1. I am using Native Android app
2. Dropdown is static
3. No, I am not recorded
4. Yes app is instrumented and its working fine for other functionality only need to know how to select value from dropdown