Hope for your immediate response.
This community consists of volunteers only doing something in their spare time. So demanding fast (especially immediate…) response is not suitable and will only turn people off from you.
I think the instructions are clear enough. If you are using Openfire with a launcher (i presume on Windows as you haven’t specified this), then: Stop Openfire, go to its installation folder, go to bin folder, create a text file openfired.vmoptions (it is imporant not to have openfired.vmoptions.txt, vmoptions has to be its extenension, not txt). Edit that file and put your desired memory min and max values. In the example they are:
Depends on your available memory and Java restrictions. Save that file. Start Openfire with a launcher. Check Admin Console. It should show changed Java Memory max value.