By my research, I found a very closely related word that denotes making something bad even worse, that is exacerbate:
1. make (a problem, bad situation, or negative feeling) worse.
But, I am not fully satisfied with the word exacerbate, as it denotes turning bad into worse. I want the word that denotes turning something good (e.g. mobile, car, computer, etc. working fairly well) into bad while trying to fix something needlessly.
The sentence, in focus, can go something like this:
- He has _____ my computer, which was already working well.
The term destroy (apart from exacerbate) might be another option for me, but it totally connotes the physical harm of anything, rather than the harm in functionality (of computer, mobile, etc).
Best Answer
If you want something as specific as "made worse while trying to fix it", you'll probably need to look into jargon and slang coined by people who have had to deal with managers who aren't as knowledgeable as they think they are. It's a common enough situation that such slang will probably be widely familiar.
That said, "messed up" (as suggested by James Wolpert) or synonyms and slang with the same meaning would be my next resort. Generally, with a computer, if there's no physical connotation inherent, people will assume a software-related meaning.
Something like "screwed up", "borked" [1] [2], "hosed", "botched up", or, if you want something that may be too specific, "reformatted".
There are probably more examples people will recognize lurking in The Jargon File (The slightly more updated online edition of The New Hacker's Dictionary) but I don't have time to go looking through it all.
Depending on the use, you could also try putting a word in sarcastic/mocking quotes, such as "reconfigured" or "fixed".