The first thing you should do is transfer the data from one of the working drives currently elsewhere in the event that a rebuild goes bad and it doesn’t maintain the data.
When you open the configurator it will let you chose from multiple devices. In the drop down menu it will list your device but have a D added to it. This means Degraded and it is the one that needs to be chosen in order for the Rebuild button to be clickable. Once you have done so it will rebuild.
However if you had ejected it and wrote data to one drive then it is out of sync and this process is the reason I recommended not swapping the drives.
If it won’t let you rebuild or you don’t have the Degraded option you will need to re-create the RAID1 from scratch, configure the unit to RAID0 then back to RAID1 and start over. (This is the reason why I suggested transferring the data elsewhere prior to starting)