Sounds like you need to write some new controls for the player!
The first question might be "do I still want my player to be able to jump using the up arrow?". If you don't mind it, then you're all good! The above advice works just fine. Add a mouse object first!
If you don't want the player to be able to jump with the arrow keys at all, you would select your player object and check under the properties for the "default controls" section. Turn off the default controls. Then, you will program in your new controls for jump, left, and right.