  We start from Rieffel data (A,f,X) where A is a C*-algebra, X is an action of
an abelian group H on A and f is a 2-cocycle on the dual group. Using Landstad
theory of crossed product we get a deformed C*-algebra A(f). In the case of H
being the n-th Cartesian product of the real numbers we obtain a very simple
proof of invariance of K-groups under the deformation. In the general case we
also get a very simple proof that nuclearity is preserved under the
deformation. We show how our approach leads to quantum groups and investigate
their duality. The general theory is illustrated by an example of the
deformation of SL(2,C). A description of it, in terms of noncommutative
coordinates is given.
