Concentration

Some spell require you to hold concentration. During the time that you hold concentration if you cast another concentration spell the previous spell ends it's effects due to the break of the concentration unless otherwise specified, other spells can be casted normally.

To break your own concentration is a free action.

To break someone else's concentration you may force them to make a check using your action if you are within 5 feet of the spell caster with the following check DC: 8+proficiency+STR/DEX or 8+proficiency+your spellcasting attribute modifier if you can spell cast. The person making the check versus that DC makes it with a saving throw using their spellcasting attribute.

You may also use your reaction to force the same check if you are within 5 feel of the target if the target is casting a spell that has a casting duration of an action or longer and has a range(spell that are exception). on it's turn, if they don't succeed the spell fails but the spellslot is wasted.

Counterspell can also be used to do both of the afore mentioned ways to tamper with spellcasting - see counterspell for more detail