How do i change the AGP slot to fit a PCI-Express GPU? without changing other things

Unless your motherboard has both slot types (very few do) you cannot.

The board will take AGP or PCI_E depending on the board.

If it is an AGP board you would need a new PCI_E motherboard.

oh thx

No problem.

Let us know if we can help further.

or does a riser card helps?

Not in this case.

AGP and PCI_E are two very different slots and interfaces and depend upon the entire motherboard.