 Loosern belt first
It may vary with engine. You might get a better reply if you specify which engine you're working on.
However, on my '02 5.4L you insert the socket end of a flex bar or into a 1/2" square hole on the end of the tensioner arm. Use this bar to unload the tensioner from the belt. While so doing, pull the belt off a convenient pulley so that the belt goes slack. My tensioner then has three 10mm bolts which secure the tensioner to the engine block.
Good Luck,
Ken | 
