In Dota Underlords sometimes you might want to see a unit to get some gold back to purchase a new unit. Thankfully you can see back any of your units at any time. To sell you units you simply have to click on them between rounds and then look to the left hand side of the screen for a prompt to complete the sale.
Click on any of your units and a green box will highlight them. You will then see a box on the left hand side of the screen that will allow you to sell them and it will tell you how much the unit is worth. Once you click the button you will see the gold immediately available to purchase and then you can use it how you choose in-between battles.
Selling units does not seem to have any penalty, even if you do it immediately. If you purchase a unit for 1 gold you can immediately sell it back for 1 gold, but the character will not be available to purchase again until you’ve completed a round or re-rolled the available units for purchase.
You will get more gold for upgraded units that they were originally worth. A two or three-star unit will be worth more than their 1-star counterparts.