Bottom bracket replacement
You need to know the type and axle length of your previous bottom bracket. If the BB is original, it is very likely BSA JIS square taper and needs to replaced with JIS square taper of similar length. You can either measure the length or just uninstall the old one and read what is printed on it. BSA bottom brackets have left handed threads on drive side. Just searching for "bottom bracket replacement" should find enough instructions.

BB-MT800 is completely different type and requires different crankset to work.