Sure. If you're working in Map Viewer Classic...
Click on Content (left side) and expand the MIL you want to edit.
Click on the blue triangle symbol to change your symbology
Click on the three dots to edit your pop-ups and labels.
If you're working in the new Map Viewer...
In the Layers pane (left side), click on the MIL you want to edit.
Observe column on the far right, there should be triangle symbol for editing symbology. You should see an option to edit pop ups and labels in the same column as well.
Sorry, wish I could use screenshots but most of my layers are sensitive info. Let me know if you need me to clarify anything!