I need to make a group of items subitems of another item. How can I do this without having to change each separately?

You change them one at a time, either in the Table view, by Pasting into Subitem of; or, you edit each and mark Subitem of; or, you Drag their little handles. Or, you export the list, change there, and reimport the list.

Nothing does a Batch, when every time we offer a tool to use, you tell us it is a Subset.

If you go to the Item list, and make sure it is in hierarchical view, then you will see a diamond in front of the items name. If you click a sub item's diamond, and drag it left, it is no longer a sub. Then you can drag it up UNDER the item you want it to be a sub of, and then drag it right to make it a sub.

In the Add/Edit view, select the item type and bring the Subitem OF column into your view. Assign the Parent, right click and Copy Down. Save.

I have the same issue. When I go to Edit > Add/Edit Multiple List Entries ... the "Subitem Of" column I want to edit is greyed out and I cannot make any changes. How can I mass update this field without having to go into each item??

Thanks for joining the thread, @lhoatmaho.

The multi-user mode maybe turned on. This is the possible reason the Subitem Of column is greyed out when editing item lists in QuickBooks Desktop (QBDT). We'll have to ensure your QBDT is in single-user mode as mentioned by @ron4. Here's how to verify it:

1. Go to the File menu.
2. Check the option Switch to Multi-User Mode. This will indicate you're QBDT is in single-user mode. If not, you'll have to click Switch to Single-User Mode option.

Once done, you can now edit the item lists under the Subitem Of column.

Also, you can follow the useful steps suggested by @qbteachmt and @chipoffblock to mass edit these item lists.

Then, you'll want to run the Item Listing report so you can keep track of the latest item info such as their price, cost, quantity on hand, etc. I'll guide you with the steps how:

1. Go to the Reports menu, and then choose Reports Center.
2. Select the List tab, and then pick Item Listing. 
3. Click the Run option to open the report.

Also, you're able to customize your reports to get the personalized details and info you want that matter most to your business. Please refer to this awesome resource for the steps on how to do it: How to Customize reports in QBDT?.

Click the Reply button if you need more help with managing your item lists or anything else related to QuickBooks. I'll be here to assist you. Take care and have a great day.