The error code theoretically tells you exactly which chips to replace. I don't have the codes handy at the moment but they can be found almost anywhere on the net. I've checked out a few 128K boards and have pinpointed exactly which chips are bad ... it's a place to start in any event, even if it is not 100% accurate as some have suggested.

Simply desolder the chips and replace them with good chips.

Out of curiosity, are these Apple branded chips? I found that almost without exception, the boards with failed RAM (which I attribute to bit-rot), are the Apple chips. Way to go Sculley, cutting those corners to increase that bonus!