Difference between "to Fill in" and "to Fill up"

[Abida Mujahid]

hello,

Abida here again

I want to know the difference between "to Fill in" and "to Fill up" which one is correct in the following context

I am going to fill in my admission form.
I am going to fill up my admission form.

thanks

[Mister Micawber]

.
These are correct:

I am going to fill in my admission form.
I am going to fill out my admission form.
.
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Native English teacher at Mister Micawber's