Currently it is a but cumbersome. You will need to edit project.pbxprox.template that is in the installation folder and change line 237 from:
IPHONEOS_DEPLOYMENT_TARGET = 7.0;
IPHONEOS_DEPLOYMENT_TARGET = 8.0;
Note that there is a similar line in line 198 (debug). It has no effect.
Edit the file while the IDE is closed.
It will be configurable from inside the IDE in the next version.