As Bob is the first row you inserted, his occupation resides in row with index 0 and column with index 1:
data.setValue(0, 1, 'Beach Comber');
In case you don't know the row index of a person who's occupation is to be updated, I suggest iterating or filtering to find all Bob rows (or the one Bob row in your case). Iteration is the 'brute force' way and goes like this
for (var y = 0, maxrows = data.getNumberOfRows(); y < maxrows; y++) {
if (data.getValue(y, 0) == 'Bob') {
data.setValue(y, 1, 'Beach Comber');
}
}
Filtering is more elegant:
var foundRows = data.getFilteredRows([{column: 0, value: 'Bob'}]);
for (var y = 0, maxrows = foundRows.length; y < maxrows; y++) {
data.setValue(foundRows[y], 1, 'Beach Comber');
}
The reference API-Doc can be found here: https://developers.google.com/chart/interactive/docs/reference#DataTable and holds a bunch a good examples.