Global changes will allow you to adjust pricing by any % required. Highlight the item (or items) you want to adjust in the line item entry window, then right click to open menu and select Global Changes.
Choose Factoring in the drop down
In Categories, choose Select
A window will open, click on the code in it (should be WTR) then click "OK"
Look in the Activity Factoring window, Equipment uses the + action, so you'll only need to change the setting for Replace.
Change the drop down to Decrease, then enter 10 in the Factor cell, and click OK.
You can also adjust all the equipment line items in a folder or multiple folders from the Grouping window in Estimate Items tab. Highlight one or more folders and right click to get into Global Changes and follow steps above. If any items you don't want adjusted are in a folder with equipment, you can check "Pause and verify" checkbox at the bottom of the window. This will let you skip over items that don't apply.