If you click Options / Keyboard Mappings, the following dialog appears:
This dialog allows you to change the keys you use for many of Abel's controls.
If you change the key for any of the controls, make sure the new key isn't being used for something else! For example, if you wanted the S key to be used for "Single", you'd also need to change the key being used for "Striking Review", since that's what S normally means.
If you want to revert to the standard Abel settings for keys, click the "Reset All" button.
Note that Abel does not distinguish between upper and lower case letters.