In Riftbreaker, there are several ways to repair buildings, the most basic of which is to press the R key. This will open the utility menu, with the Repair tool selected. All you have to do from there is click on the buildings that need to be repaired. Alternatively, you can bring up the build menu by pressing CTRL and then clicking on the building you want to repair. You’ll see some menu options appear, and one of them will be repaired.