Intentionally end the spell by dropping concentration
The conjure elemental spell description says (emphasis mine):
The elemental disappears when it drops to 0 hit points or when the spell ends. [...] If your concentration is broken, the elemental doesn't disappear.
So intentionally ending the spell would dismiss the elemental. This is also mentioned in the Sage Advice Compendium ("Can a spellcaster dismiss a spell after casting it?", p. 13):
You can’t normally dismiss a spell that you cast unless (a) its description says you can or (b) it requires concentration and you decide to end your concentration on it.
So ending a concentration spell is done by dropping concentration:
Some spells require you to maintain concentration in order to keep their magic active. [...] If you lose concentration, such a spell ends. [...] You can end concentration at any time (no action required).
Note that this does not count as your concentration being broken; the rules further say:
The following factors can break concentration:
- Casting another spell that requires concentration. [...]
- Taking damage. [...]
- Being incapacitated or killed. [...]
This is supported by a relevant unofficial tweet by lead rules designer Jeremy Crawford:
Willingly ending your concentration on a spell in D&D isn't the same thing as having your concentration broken. In the rules on concentration (PH, 203), willingly ending your concentration is purposefully not on the list of things that can break your concentration.