The last official update the Galaxy S3 received was Android 4.4.4 KitKat so there is no official update to Android 5 Lollipop. The only way to get a newer version of Android on that phone is by rooting your phone and installing a custom ROM. There are many guides online on how to do this - e.g. these guides by CNET, Gadget Hacks, and DroidViews. A popular custom ROM for the Galaxy S3 is LineageOS. You can find more custom ROMs on the xda-developers forum. Note: if you are an inexperienced user, I do not recommend you do this, as you can lose all files or damage (brick) your phone.