1)You can undo the parameter in the following 2 ways
a)Use the following command to configure the RMAN parameters
RMAN>configure channel device type disk;
Dont give the format and the destination.
b)In the RMAN script, forcibly use the following command to overwrite the RMAN configuration done.
run
{
allocate channel c1 type disk;
allocate channel c2 type disk;
allocate channel c3 type disk;
backup database format 'Z:\RMANTEST\%d_DB_%u_%s_%p';
.
.
.
}