Hi @Shervan360
You can use find function (Ctrl+F shortcuts) to find all ".Pages" in current project and then replace them to ".My".
Refer to the following steps:
1.Create a Razor Page application. Change the root directory and change the Pages folder to "My".
2.Press the Ctrl + F shortcuts, the in the "Find..." textbox, enter the ".Pages", and select the "Current project" option, after that select the "Find All" option.

After that, it will show all files which using the ".Pages":

3.To change the ".Pages" to ".My", you can expand the search panel, and in the replace textbox, enter the ".My", after that click the Replace All icon to replace all text.

After that, it will show the following notification window.

And if you check the relates page, you can see the related page's namespace has already changed.

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Best regards,
Dillion