Before I try to answer that, first I should say that I have read about 'correcting' POI in several articles through the years, and also seen reference to a factory doing such. Unfortunately I do not have such readily available to document or refer you to. I have also done this myself, so I do know it can work.
The best way is to have three 'half blocks', wood is OK, two to support the barrel and one to apply pressure. A large hydrolic press is convienent, but a heavy benchtop and a large c-clamp is enough. Some forethought about how much correction left/right/up/down is required; for instance, if you need 6" right and 3" up it would be best to do both right and up with one corrective bend rather than two separate corrections.
It is also worthwhile to look the barrel over carefully to ensure if it has been already bent. The best way to do this to to view down a clean, bright bore to a reflected light (say, off a white wall); you will be able to define the 'concentric circles of light' within the bore and see if it veers off to a side. If you think you see such, rotate the barrel to see if the concentric circles follow the rotation.
Once you determine a bend, consider if straightening it will agree with your correction or aggravate it. Think this well through!!!
When sure of making a correction, set a half block at either end of the barrel, one centered up (or, if a previously bent barrel, at the bent location; think of straightening a nail) and use the c-clamp to apply pressure, one end over the half block, the other under the benchtop lip. Be advised that the steel has a memory and will wat to return to its 'original' state, so you need to apply more bend than you think neccessary to achieve a 'take'.
You can measure before and after, and a masking tape reference point might be useful on the barrel pressure point; but the real proof is in the shooting, so dont go nuts trying to keep track of 'the measurements' unless you just cant resist.
