Updating "Extra" Records

This is sort of the reverse of a question I asked here:
https://www.experts-exchange.com/Microsoft/Development/MS-SQL-Server/Q_28064255.html

I have a table which looks something like this:

Job    Phase      Cost
1234   ABC        123.45
1234   DEF        123.45
1234   GHI        123.45
5678   ABC        345.67
5678   DEF        345.67

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What I need is an update query that will remove the extra cost leaving only one cost per job.  In other words, I need the result to look something like this:

Job    Phase      Cost
1234   ABC        123.45
1234   DEF        NULL
1234   GHI        NULL
5678   ABC        345.67
5678   DEF        NULL

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I don't care which phase ends up with the cost (and which phases have their costs deleted), all I want is to have one cost per "Job" no matter how many phases each job has.

Although it probably shouldn't matter, the above is merely an example. The actual table has several other fields, none of which should be of any concern for this.

TIA