I've accidentally un-stalled a system app & having issues on rooted S5 Mini, what is the best way to re-install an OS?

[AC Question]

I've accidentally un-stalled a system app & having issues on rooted S5 Mini

Rooted my S5 mini (works phone), uninstalled apps via & Jumobile app remover & Titanium backup which has failed like a good 'un. Foolishly, I've reset factory settings (rookie mistake I know) and now have got diddly squat. Best way to re-install OS?