Issues in Journal Entry customization

Hi ,
1- Can you tell me Where I can find html and url files of Journal Entry in ERPNext?

2- How can I change the currency symbol from standard field to another symbol that change by select from select link by using client script

3- Why I Can’t change value of standard field permanently , when we change it from client script (on submit) when we reload the page , all our changes are deleted ?

4- How can we update from the code (python)
where should we update and How can we run the program after changes.

please help us