Hey Charles,
You can achieve this by following these steps
Firstly change the button's text to expression 
then create a local variable of text type and set the default text you want to set on the button.
And set this local variable to expression.

then you have to set on click client action and Assign the new text to a local variable.
I think that will resolve your issue.
Thanks and Regards,
Abhishek Paliwal