Just change value of field "order" in corresponding screen.
For instance, in your exemple, change order in "Milestones types" so that "fixed milestone" is the fist one.

Thanks for your quick response.
in activities and milestone types i did not find the planning values to change . where i can change it?

in activities and milestone types i did not find the planning values to change

I don't understand the request.
Please explain.

You said :
" For instance, in your exemple, change order in "Milestones types" so that "fixed milestone" is the fist one. "

I did not find "fixed milestone" or "float milestone" in "Milestone types" to change the order.

I found that i have to change the values of ordersort in "planningMode" table in database

if some milestones are already created will they be affected by this change of order value?

OK.
Sorry but I misundestood.
What I explained works for type list.
For the "planning mode" list, you have to act directly in the database : change sortOrder in the database table planning mode for applyTo='Milestone' will do the job

if some milestones are already created will they be affected by this change of order value?

No, they won't

Thank you!!