Will be English thread bottom bracket. If you have an old school bottom bracket with loose ball bearings, then simply disassemble it (leave fixed cup in the frame), clean out the old gear, replace the balls (very cheap), pack with new grease (any automotive bearing grease will be fine), and reassemble (adjusting it takes some touch, expect to take a couple of tries). Those bottom brackets will run essentially forever. Unless it is rusted out or you've worn through hardened bearing surfaces (really unlikely), you don't need to replace it. A modern sealed bottom bracket is not a better component and not an upgrade. Overhauling loose ball hubs and bottom brackets gets faster as you get more practiced, the next time you overhauled those hubs it'll take ten minutes per hub. Ditto the bottom bracket.