Is it possible to adjust tabel headers of GeoJSON files in ArcGIS?
If yes, which max length has a row header?
In QGIS there is no possibility to edit a GeoJSON table.
In QGIS you can just transfer the layer to a memory layer, edit it, then transfer it back to GeoJSON. (or use a shapefile instead of a memory layer).
However, if I remember correctly, Table Manager (as mentioned by matteo) lets you edit these headers and the rest of the structure, for many files including GeoJSON. https://plugins.qgis.org/plugins/tablemanager/
In order to do this in ArcGIS you need to convert the file to a shapefile or geodatabase (or use a plugin), then edit it as that format, then re-export as a GeoJSON.