There are two ways to scan a single TG; one is to "hold" on that TG and the other is to only have that TG turned on or programmed.
To change the TG that you are holding on, press FUNC TRUNK. Use the T, UP and DOWN keys to find the new TG you want to hold on. Press FUNC TRUNK again. After a moment, the scanner will resume "ID HOLD MODE".
Note that using ID hold mode in this manner is not ideal. One problem is that when the control channel changes, the scanner does not follow it. If you really want to follow only ONE TG, I recommend turning off the sub-groups of the TGs you don't want and locking out all the TGs in the enabled sub-group you don't want. Not elegant, but clearly the only fully funcitonal approach.