How to update upper-bound of tstzrange ?

Could someone kindly help me out with the correct syntax ?

My first thought was the below but that doesn't work:

update foo set upper(bar_times)=upper(bar_times)+interval '1' hour where bar_id='abc';
ERROR:  syntax error at or near "("
LINE 1: update event_sessions set upper(bar_times)=upper(bar_ti...