I can't find anything specific for Samsung laptops but BIOS batteries on other makes are a complete pig to replace, they are on the motherboard and require an almost complete disassembly. If you have just had it repaired then I'd take it back to the repairer and tell them that you'd like it working properly, it really isn't something to be taking a shot at if you don't know what you're doing, without proper anti-static precautions you could damage essential components.