Hello I need help,
I'm very bad at coding.
I need one "field value + 1990"in pop ups. Would be the best if I wouldn't need to change other fieldname. Can I recalculate existing attributes?
Yes, you can use Arcade to calculate fields: https://doc.arcgis.com/en/arcgis-online/manage-data/calculate-fields.htm
But if you only need it in the popup, and not as an actual field, just add an expression:
$feature.your_field + 1990
That will literally return the value of your field with 1990 added to it.
Arcade expressions added to your popup configuration become available in the "fields list", so you could display the output of the expression alongside or instead of other fields.