How to replace characters in visual block selection

I use vim 9.1.0016 on Windows 10.

In second column I would like to replace all A characters with X characters.

Sample:
AAA AAA
AAA AAA

After change:
AAA XXX
AAA XXX

Above is very simple sample. In my case I have text of words, not just letter A.

I did:
My cursor after opening a file is at first character in top left as expected.
1. Move 4 characters to the right: 4l
2. To get into visual block: CTRL+V
3. Select current and line bellow: j
4. Select text to tend of line: $
5. Now second column is selected. I would like do the substitution with: :s/A/X/g
And now I get:
XXX XXX
XXX XXX

How to substitute characters inside visual block?
Regards

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