Updating column of type tsrange

Hi
I want to update one of the bounds of tsrange data type in a column.
Eg
Suppose I have a column time_range of type tsrange in a table with one row having data (a,b) for time_range column.
I just wish to update the b part of this range data without writing something like
update tablename set time_range = (a,b')
(b' -- new value of b)

Kindly let me know if its possible.

Regards
Ajay