The original caster will flee.
Here's the Ring of Spell Turning: (DMG, p.193):
While wearing this ring, you have advantage on saving throws against any spell that targets only you (not in an area of effect). In addition, if you roll a 20 for the save and the spell is 7th level or lower, the spell has no effect on you and instead targets the caster, using the slot level, spell save DC, attack bonus, and spellcasting ability of the caster.
Addressing your sub-questions:
Can you specify another option for the spell (halt, approach, etc.)?
No. The target of the spell has changed, nothing else. The wearer of the Ring doesn't control anything about the spell.
From where will the cleric flee?
The Flee option on command is:
The target spends its turn moving away from you by the fastest available means.
Since the new target and "you" (the caster) are now the same person, technically the cleric would spend their next turn fleeing from themselves. It's up to the DM to decide what that means; having them flee randomly or from the original spell's targets seem to be reasonable options, as is declaring that the command can't be obeyed, and thus has no effect.
In case the spell was cast using a higher level (affecting a second creature), could be the spell be returned?
No; the Ring of Spell Turning affects spells that have only the wearer as a target, and none others. It would have no effect at all against command cast on multiple targets (not even advantage on the saving throw).
Suppose the returned spell required concentration or had a dismiss option. Could the original caster finish the spell by ending concentration or using the appropriate action to end the effect? When could they do this?
Yes, they are still the caster, so they can end concentration on the spell or take an action (if applicable) to end it. Technically concentration can be dropped at any time, but most DMs would probably not allow this until after the initial effect of the spell had resolved. If an action is required to end the spell, then the caster follows whatever rules are specified in the spell description.