input - Make a typed character replace next characters on ASP.NET Textbox -

i trying found script resolve problem.

i need number user enter, replace next number existing textbox. example:

my default textbox value 00:00 , max length 5. need next number user inserts replace other next! in case showing in example (00:00), if insert 1 , after 3, content of texbox this: 13:00.

max length 5, , @ way textbox don't let me insert more text! need make new text inserted replace existing others inside textbox.

sorry bad english guy. , atention