My Mobile Is A Galaxy Y. The OS is 2.3.4 Gingerbread. what i wanna do is Update it to 2.3.5 / 2.3.6 . By Updating Will I Loose My Applications Which Are On Mobile?
It depends on the update. Over-the-air (OTA) updates do not wipe your data, so you should definitely accept such an update if there is one. Updates which you have to flash yourself (e.g. using Kies) usually do wipe your data, but the documentation that comes with the update will confirm whether it does.
Installing a custom (third-party) ROM almost always requires you to wipe user data.