How you do this will depend entirely on whether you want those changes to stick if you quit and reload JAWS, or whether you want them "for this session" only.
Bring up the keyboard shortcuts document and the method for doing either temporary, or permanent, on the fly changes in speech rate are there.
Brian - Windows 10 Pro, 64-Bit, Version 1903, Build 18362
Many of the insights of the saint stem from his experience as a sinner.
~ Eric Hoffer