You really do not give enough information to enable a precise answer.
If you mean that the instance of SQLServer has it's date format set to mm/dd/yyyy and you want it to be dd/mm/yyyy then
this[
^] might be useful.
If you mean that you have a procedure/function that receives a string in mm/dd/yyyy format then
this[
^] will help.
For anything else, please edit your question to say
exactly what problem you are having.