1. The highest an S5 updated to was 6.0.1, which is still pretty old.
2. That update was released around December 2015. If you check in Settings (look for update) you should find a way to update to that.
3. Straight Talk is a mobile virtual network operator, not a carrier – phones come in models by carrier (AT&T, Sprint, TMobile and Verizon – and Straight Talk uses any of them), so to really give you any answers, we need the model number of the phone (on the label under the battery) – SM-900A, SM-900T, SM-902VL, etc.
4. Straight Talk doesn’t issue updates – you’ll have to find an update yourself (SamMobile, XDA Forums, etc.) and use Odin to flash it (see [Samsung] How to flash Stock ROM via ODIN).