how to update field in databse using a!update

I have created expression rule to find data like {{changeid:"1234",prioritystatus:null}}

now i want to update this list of dictionary using a!update how can i do that? i want to update that field prioritystatus. I am doing this by selection of one prioritylist in dropdown box.

I have done this in user interface and on changing this status it will update the above field in this list of dictionary. But it gives error.

I have done like this:

a!update (data:{{changeid:"1234",prioritystatus:null}},index:"prioritystatus",value:1)

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