These are some great questions, and a topic that easily separates the beginners from the pros.
You have to start at the top on this item.
Is it worth keeping the current tenants? Do they pay market rent? Do they take care of the place? Are they trending in a bad direction, or is it really just a temporary situation?
If the answers to these questions are not favorable, you might want to think about inviting them to leave.
If the answers are favorable, then set a deadline for them to start paying on time again. If they meet that date, waive the late fees. If they miss it, make all late fees and rent due immediately.
If you can easily replace the tenant, be a little tough. If you can’t, be a little tough, but work with them while they get things back on track.