How to update a object without a new variable.

In Appian Variables are designed to be permanent once created - you can't alter them after initialization. To make changes, you must build a brand new variable that incorporates your modifications while leaving the original intact.
Hope you are expecting this when you want to append list...

a!localVariables(
  local!originalList: { 1, 2, 3, 4 },
  local!listToAppend: { 5, 6, 7, 8, 9, 10 },
 { local!originalList, local!listToAppend }
)

you can use append() function if you want t add at the end of list always or use insert function if you want to insert at specific indices/index

insert({1,2,3},{4,5,6},4)

append({1,2,3},{4,5,6})