You can update only Selected tasks from a project by going to menu Project – Update Project, select the “Set 0% – 100% complete” or “Set 0% or 100% complete only” option under the “Update work as complete through:” and select a date from the date picker. From the “For:” section check the “Selected tasks” checkbox and click OK.
Note: If you don’t specify a date, Project Plan 365 uses the current or status date.