Creating a new list with new second element

The following works, I think

data = {{1, 34}, {2, 54}, {3, 66}, {4, 77}, {5, 92}};
data[[2 ;; 5, 2]] = Differences[data[[All, 2]]];

Output when data is called is

{{1, 34}, {2, 20}, {3, 12}, {4, 11}, {5, 15}}

Adjusting for specific lengths (dimensions of a submatrix to be replaced) is straightforward, e.g.

data[[2 ;; Dimensions[data][[1]], 2]] = Differences[data[[All, 2]]];

ClearAll[f]
f = SubsetMap[Differences @* Prepend[0], {All,2}]

Example:

data = {{1, 34}, {2, 54}, {3, 66}, {4, 77}, {5, 92}} ;

f @ data

  {{1, 34}, {2, 20}, {3, 12}, {4, 11}, {5, 15}}

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