Can you please elaborate a it more? I am assuming you have got a dropdown on your page and some other data that is based on the value chosen in the dropdown (e.g. a dataview)?
You can add an onchange activity on your dropdown, where you refresh the object needed(e.g. your page parameter to refresh the page).
Hope this helps
Check the Constrained by function. See the documentation here: https://docs.mendix.com/refguide/reference-selector#xpath-constraints