How to modify a 433.92 MHz signal to a 434 MHz signal?
"How can I modify a 433.92 MHz signal to a 434 MHz signal?"

Don't. Frequency-wise, what you bought is fine.

The difference is only 0.02%, the electronics would have a very hard time to distinguish between those two frequencies, especially the very cheap electronics found in typical consumer products. So the documentation often rounds the value a little (or just chops off the .92 part and calls it 433). I very much doubt that your oscilloscope would be able to tell the difference, so the 434.000 you measured is probably 433.92 (or at least meant to be).