Do i have to upgrade it or its better not to??what do i advice me??
Welcome to iMore. The latest iOS update is iOS9.0.1, to upgrade or not is really a decision you would have to make. Apple always recommend that iPhones should run the latest software so the device can run a peak performance.
In general terms, it's best to upgrade to the latest version of iOS. However, there are times when it may not be the best move. And since iOS 9.1 is still beta software, this might be one of those times. With that said, I've been running the 9.1 beta for weeks and have no issues.
Since you are being "asked" to upgrade, you must be enrolled in the beta software program... so at one point you had to have opted into that process.
So should i upgrade it or not???