A lot of libraries can be installed with the library-manager inside the Arduino-IDE.
Though if the library is not found inside the library-manager you can do a google-search if the library is availabe on GitHub
search with google for
Github libraryname.h
If you are in the main-folder of the GitHub-library
in the upper-right you see a green button code
click on this button and choose download ZIP
Save the ZIP-file to your harddisk
In the Arduino-library click Sketch - include library - add .zip-library
EDIT: choose the ZIP-file you just saved before to your harddisk
If the installing was successfull the IDe will show a message at the botton
best regards Stefan