Hi
@nishan
If I understand correctly you have removed the factory mirror arm from the windscreen. You will be left with a metal button glued onto the windscreen like this:
View attachment 7867825
The factory arm and replacement arms attach to that button by metal fingers. To remove the arm you twist it counter-clockwise until it comes off. To refit to the windscreen you do the reverse. You might need to do that action a couple of times to work it out. Be patient. Don't force anything.
View attachment 7867826
Not a great example, but the mount and method is the same as
this Audi mirror. Twist and pull to remove. Line up and press on, then twist to refit.
The factory arm is very tight. If you cannot turn it by hand you can protect the arm with something non-marking (I used a leather work glove) then use a pair of multi grips to provide extra leverage.
If all that fails any decent windscreen replacement shop will help you out, probably at no cost.