In Zabbix Version 4.0.x it is only possible to replace the whole set of tags for triggers.

It would be a nice feature to have the following possibilities modifying dedicated tags.

replace = replace only the tag and value for the given tag-name
replace all tags = replace the whole set (like how it is now) with new given set
add = add a tag with given value to the existing tags
remove = remove only the given tags from the tag set

This is already in non-LTS Zabbix 4.2.