Here’s a quick walkthrough of one way of doing things.
First I created a new project, and I filled in the Composer and Arranger fields in Project Info.
If I look at the score in Engrave mode, I see the composer’s name, but not the arranger’s.
On the right, under Page Templates, I double-click the First template so I can edit it. I can see that it has a token for the composer’s name on the right, and also a token for the lyricist’s name on the left. I’m going to replace the lyricist’s name with the arranger’s name.

It got cut off in my screen capture, but when I right-click in the text frame, I get a menu with all of the available tokens, and under Project Info, I chose Arranger.
Notice that I clicked the L->R button, to make sure that changes on the left-hand template get propagated to the right-hand template, then Apply and Close.
That was just for the score. To do the same thing to the parts, I switch to a part in Engrave mode. Under Page Templates, on the right, it now says Default Part instead of Default Full Score. Then I carry out the same operation as above: double-click to open the template, replace the Lyricist token with the Arranger token, Apply, and Close. Now my Trumpet part looks like this:
Because I made the changes to the page template, instead of directly to the part, that field will now show up on the first pages of all of the parts in the project. (Unless I’ve already made other edits to the parts that result in a red triangle showing up in the Pages area – that indicates an override, and we can deal with that separately, if you’re in that situation.)