1- log into github.com
2- "Fork" someone else project by clicking on Fork
3- Now you have a copy of the project ( fork )
4- Download it to your laptop ( using github desktop)
File >> Clone repository
and then select the repository ( same as the one on github.com ) 
5- a new diretory has been created localy
6- modify the file ( localy)
7- github desktop show the changes and commit the changes ( on the master branch )
8- push the changes ( synchronize )
After:
9- check on github.com the changes
10- pull a request to the original software developper