If you are referring to updating from Better Backpacks v28 to Better Backpacks v28.1, it's just as simple as installing v28.1 as if you didn't already have the mod installed. For most mods, unless they say otherwise, you can install over the top.
I don't know if Better Backpacks has a config file as I don't use it myself, but if it does and if you modified the config file, then you may want to either back it up or make note of the changes, because updating the mod will overwrite those changes and put them back to default.