It may just be due to English not being your first language, but you do need to understand that the Stack Pointer is part of the 8051 hardware - its address cannot be changed!

What you can change is the starting address of the Stack - and you do that by changing the value that is stored in the Stack Pointer.

In other words, you change the value of the Stack Pointer - not the address of the Stack Pointer.

I know that sounds pedantic, but you will get yourself seriously confused if you don't have a very clear understanding of the difference between the value (or content) of a Pointer - any Pointer - and the address of the Pointer...