One way to do it is to leave the bolt in the rifle, make sure it is unloaded, dry fire it, and then use an allen wrench in the hole in the back of the bolt assembly screw.

Or you can take the bolt out, manually release/decock the firing pin, using the strong wooden edge of a workbench to push against, and then use your allen wrench.

Usually the first time i disassemble a new bolt, I place it on the carpeted floor and step on the allen wrench as the bolt handle pushes against the floor acting as a stop. After that, I can do it while it is still in the rifle.