Answer by Oleh Berehovskyi for How to change UI Button label in controller?

I have recently tried to change a label of <ui:button> by myself using component.set() method:

component.find('Btn1').set('v.label', 'New Label');

and it absolutely works. Can you please provide an error message, because it should work and probably the issue is caused by something else?

Since of API version 47.0 <ui:button> is deprecated. Use <lightning:button>, <lightning:buttonIcon>, or <lightning:buttonIconStateful> instead.