Halcifer

I'm new to the Mig-21 and have a question about the Grom missile. If I use radar beam riding mode and decide to lock a ground target but the pipper is initially off, how do I go about unlocking or adjusting the the pipper? I know this can be done when the target isn't locked and just by manually riding the pipper, but I can't figure out how to disengage it once locked. Thanks,

I also started experiencing this after the 1.5.7.8899 update.