If I update my Iphone to iOs 6.1.2 Will i lose all my photos?

Will all my photos and memory et erased

Not if you back it up to itunes. Just click on backup and restore option before you update. Once you update, your iphone will be blank but your computer saved the backup so it will load up your phone with your pics and songs.

No when you update your iPhone whether it be by plugging it in to your computer and using iTunes to update it or if you just do an over the air update, all of your photos should be fine as well as all of your other data.
It is always a good idea to back up your iPhone before installing these updates though just to be safe, though it is not necessary to do so.
To back it up just plug it into your computer and sync it with iTunes.

To do an over the air update go to settings, general, and software update, your battery must be over 50% or it must be charging to install.