Anyone know a thing or two about fault codes... (First post here, can't find too much on the below)

My 320i E90 2005 has codes 5DE0 & 5DE1

Mechanic has told me I need to replace the ECU and get the new one installed... I'm not quite sure on this yet..

Have done some homework and seems to be a recognised fault that can be dealt with but can't see where people are replacing the ECU?

Assistance here if anyone in the know has some

(I've had the break pads replaced on the rear wheels)