How to modify project expenses ?

In ProjeQtOr, you can define Expenses in Financial menu : project expenses or individual expenses (when liked to a resource)

For "expense" of resource work, in ProjeQtOr it is called Cost.
So there is a difference between "expense" and "cost" (you'll see 2 lines on project summary).

You can define the validate "cost" for each activity, but no validated "expense" for each activity, as expenses are not linked to activity.
You can define validate cost and expense at project level.
Pay attention that validated data may be consolidated from tasks (elementary activity) to project, or not, depending on global parameter "consolidate validated work & cost"

If "validated" cost (or work) is greyed, it means it is consolidated from sub-tasks.