runterkommen
to come down
To move down from a higher place.
Komm sofort vom Baum runter, das ist zu gefährlich!
Come down from the tree right now, that's too dangerous!

to come down / calm down / become run-down
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to come down
To move down from a higher place.
Komm sofort vom Baum runter, das ist zu gefährlich!
Come down from the tree right now, that's too dangerous!

'Runterkommen' (to come down) is a separable verb. The prefix 'runter' comes at the end in a main clause. In subordinate clauses, the verb stays together.
Bitte komm sofort vom Dach runter!
Please come down from the roof right now!
Since 'runterkommen' (to come down/calm down/become run-down) describes movement or a change of state, the perfect tense is always formed with the auxiliary verb 'sein' (to be). A common source of mistakes is using 'haben' (to have).
Nach dem anstrengenden Tag bin ich auf dem Sofa endlich runtergekommen.
After the exhausting day, I finally calmed down on the sofa.
The prefix 'runter' is the colloquial short form of 'herunter' (down). In a more formal text or conversation, you would use 'herunterkommen' (to come down).
Der Bergsteiger ist sicher vom Gipfel heruntergekommen.
The mountaineer came down safely from the summit.
In the meaning 'sich beruhigen' (to calm down), the verb is often combined with the word 'wieder' (again): 'wieder runterkommen' (to calm down again). It describes the process of returning to inner calm after excitement or stress.
Atme tief durch, dann kommst du schnell wieder runter.
Take a deep breath, then you'll calm down again quickly.
When 'runterkommen' (to become run-down) describes decline, it usually refers to buildings, neighborhoods, or facilities. Here the verb has a clearly negative and derogatory meaning.
Seit der Schließung der Fabrik ist der ganze Stadtteil ziemlich runtergekommen.
Since the factory closed, the whole neighborhood has become pretty run-down.
to switch off / to unwind
'abschalten' and 'runterkommen' can both mean that you become calmer and relax. 'abschalten' mainly means mentally distancing yourself from work, problems, or stress. 'runterkommen' is colloquial and fits especially when someone was previously very stressed, agitated, or angry. Learners may confuse the words because both are used after a tiring or intense moment. In practical terms: you use 'abschalten' to clear your head; you use 'runterkommen' when strong tension gradually decreases.

Nach dem Streit braucht Paul etwas Zeit, um wieder runterzukommen.
After the argument, Paul needs some time to calm down again.
Vor der Reise möchte ich noch einen Abend zu Hause abschalten.
Before the trip, I want to spend one more evening unwinding at home.