Run a separate build for that contest, or lock the other lineup groups while you set exposures. Separate builds are the cleaner option.
Separate build. Let One Group Per Contest create your groups, then delete every group except the contest you want to work on. Set your exposures and rules, build, and save. Only that contest gets filled, and you make another build for the rest. This is what we recommend. Exposures live inside a build, so there's nothing to undo afterward, and players who want different exposures for different contests usually want different rules for them too. The "How do I build lineups for a specific contest?" FAQ walks through it.
Locking. Exposures apply to every unlocked group at once, so locking the other groups narrows a change to one contest. Lock the groups you don't want to touch, set your exposures, and only the open group updates. Note: Exposures only affect a group with more than one lineup. A single-lineup group ignores them unless you set 0% or 100%.
The catch is unlocking. A locked group is protected only while it's locked. Any exposures still set when you unlock it apply to it immediately. So if you want a different set of exposures for each of three groups, you have to work in sequence: lock groups two and three, set exposures for group one, lock group one, clear the exposures, unlock group two, set its exposures, lock it, clear again, and so on. It works, but one step out of order rewrites a group you thought was finished.