if it is like my friend's, the one in her Buick did not seem adjustable ... it was an '89, I might have to look at it again ...
If the D or V ring the belt runs through is mounted or fastened in a visable vertical "slot" it is adjustable. If it is, the fastener, which may not be visable is a pan head "allen" type screw. Be prepared with a long "allen" wrench, because it is torqued to about 85 ft/lb, and requires a fair amount of strength to loosen without a torque wrench with an allen type adapter. Most cars after 1995 are adjustable. An '89 Buick might not be.