See now, what you need to do there is take the 3x magnifier off, put it in a draw, go play airsoft and sell the magnifier in 6 months. This will save you the hassle of lugging it around all day just to discover that it's 3 parts of fuck all use.
Alternatively, the should be elevation and windage adjusters on the magnifier or possibly on the mount base that will help you re-centre the reticule. Unfortunately, there is EVERY likelihood that when you first flip the magnifier to one side and back again it'll be off centre again because NONE of the cheap magnifiers are actually any good (please see my previous comment above).