Change boundary label of a point on the boundary

Hello everyone,
I am new to Fenics and I am stuck on a problem.

Basically, I would like to change the label of a point on the boundary.
The boundary is a semi-circle. The basis is marked as 1 and the curved part is marked as 2.

I would like to change the label and mark all of the boundary points as 1.
I am using Fenics 2019.