inserting constant at a specific address of .hex file
The immediate question is: why do you feel the need to do that?
But "how to do it in Keil" is a Keil question - nothing specifically to do with STM32.
The Keil documentation is available here: ARM Product Manuals
Also check out their support & learning materials:
Cortex-M Learning Platform
Keil Product Support
Keil Embedded Bookshelf
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