How can I edit "Category" in IPTC-IIM in Bridge CC 2018?

Keep in mind that the legacy IPTC:Category entry was officially only 3 characters.

Guideline for mapping Category Codes to Subject NewsCodes

The Photoshop:Category entry will allow more characters to be written, however when Adobe software also writes out the legacy metadata to the IPTC:Category field the entry will be truncated to 3 characters. The “problem” with many “official industry specific” metadata fields is that they are not correctly used by people outside of the specific industry… so there may be a potential discrepancy between these two fields, depending on the software used to enter and or view this data.

You could use a script...

#target bridge   
 if( BridgeTalk.appName == "bridge" ) {  
Category = MenuElement.create( "command", "Edit Category","at the end of Thumbnail");
}
Category.onSelect = function () { 
if (ExternalObject.AdobeXMPScript == undefined)  ExternalObject.AdobeXMPScript = new ExternalObject("lib:AdobeXMPScript");
thumb = app.document.selections[0];
app.synchronousMode = true;
var xmp = new XMPMeta(thumb.synchronousMetadata.serialize());
Cat = "";
if( xmp.doesPropertyExist(XMPConst.NS_PHOTOSHOP, "Category") ){
    Cat = xmp.getProperty(XMPConst.NS_PHOTOSHOP, "Category");
    }
var win = new Window("dialog", "Set Category");
win.p1= win.add("panel", undefined, undefined, {borderStyle:"black"}); 
win.p1.alignChildren="fill"; 
win.g10 =win.p1.add('group');
win.g10.orientation = "row";
win.g10.alignment="left";
win.g10.st1 = win.g10.add("statictext",undefined,decodeURI(thumb.name));
win.g20 =win.p1.add('group');
win.g20.orientation = "row";
win.g20.alignment="left";
win.g20.st1 = win.g20.add("statictext",undefined,"Category:- ");
win.g20.et1 = win.g20.add("edittext",undefined,Cat);
win.g20.et1.preferredSize=[100,20];
win.g200 =win.p1.add('group');
win.g200.orientation = "row";
win.g200.alignment="center";
win.g200.bu1 = win.g200.add("button",undefined,"Update Field");
win.g200.bu2 = win.g200.add("button",undefined,"Cancel");
win.g200.bu1.onClick=function(){
if(win.g20.et1.text == ""){
    alert("No Category has been entered!");
    return;
    }
win.close(0);
xmp.setProperty(XMPConst.NS_PHOTOSHOP, "Category",win.g20.et1.text);
var newPacket = xmp.serialize(XMPConst.SERIALIZE_USE_COMPACT_FORMAT);
thumb.metadata = new Metadata(newPacket); 
ExternalObject.AdobeXMPScript.unload();
ExternalObject.AdobeXMPScript = undefined;
}
win.show();
};