To use substitute, you have to be at the level of the symbolic expression underlying the scalar field kr. The latter being returned by the method expr(), it suffices to write
kr.expr().subs(m==10)

To use substitute, you have to be at the level of the symbolic expression underlying the scalar field kr. The latter symbolic expression (in the default chart) being returned by the method expr().expr(), it suffices to write
kr.expr().subs(m==10)