Edit Custom Field Script (onchange) value

I use your Community Builder component, and i create a custom field for registration and profile.
but i want to add some function, lets says;
1. i created a custom field (drop down single select) named provinces
2. i also created a custom field (drop down single select) named city

what i want is;
when i select some value on Province field, the value for city will change according to what i pick/ select on Province Field.
example;
when i select AAA on Province field, in the city field, there are A1, A2, A3, A4, A5
when i select BBB on Province field, in the city field, there are B1, B2, B3, B4, B5
when i select CCC on Province field, in the city field, there are C1, C2, C3, C4, C5

how can i do that?

CB Query Field and its Update On functionality can provide chaining like that. Basically 1 query grabs the countries and when it changes it updates provinces which queries for provinces of the selected country, etc.. Aside from that you'll need to custom code your own fieldtype plugin to do it.