We have an existing limesurvey project of version 1.9. Now, we want to upgrade this to latest stable version. Could anyone please answer my bellow questions:

Q-1: What process should be followed for this version upgrade ?
Q-2: Is this upgrade hamper the existing data/database ?
Q-3: Should we need to upgrade the database also ?

Many thanks for your help !

Md. Al-Hasan

Q1: I would install LimeSurvey in a new folder first. You will no longer can use old templates when upgrading. Many of workarounds will no longer work. Depending on what version you choose (2.6Y.Z or 2.6.ZLTS).
You will need some time to adapt to the new version in terms of GUI.

Q2: The database will be changed during a upgrade. So backup first.

Q3: Depends on what database/version is running now. You might need to update PHP too.

Requirements: manual.limesurvey.org/Installation#Make_...rvey_on_your_website

Different approaches to upgrade LimeSurvey can be found here (Try a fresh installation first is my recommendation!):
manual.limesurvey.org/Upgrading_from_a_previous_version