(1) Explain your problem, don't simply post "This isn't working". What were you doing when you faced the problem? What have you tried to resolve - did you look for a solution using "Search" ? Has it happened just once or several times?
(2) It's also good to get feedback when a solution is found, return to the original post to explain how it was resolved so that more people can also use the results.
several ways here
go to the projects directory
highlight your project
go to the calculations tab and on the right make sure that on the right under resource assignments that recalculate actual units and costs when duration % complete changes
now go to the defaults tab there and under defaults for new activities that the percent complete type is set to duration
now when you update and activity lets say that is 10 days original duration and you progress it to 50% complete then the remaining duration will automaticaly be 5 days.
if you are using physcial % complete then go tot tools
gobal change
then New
If Parameter Is Value
Where physical % complete is not equal to 0
Then Parameter Is Value
duration % complte = Physcial % complete
then on the right select change