I want to write a pure function to change the key's value of an association, it must return the new association, so it can be mapped to a table of associations.
The AssociateTo can change key's value of an association, but it seems not working. I don't know why, perhaps because it is not a pure function?
So if I have a pure function doing the same thing, this job can be done very well.
Can you help me? Thanks!

Append.AssociateTois for changing the value of a variable. You have no variables. $\endgroup$Merge[Last], in even more general case 102466, but here, in addition to what Szabolcs said, you can<|#, "key"->value|>&/@data$\endgroup$<|#, "key"->value|>works well. $\endgroup$