Right click on package.appmanifest and select "View Code"
Find the line
<Identity Name="XXXXX-XXXX-XXXX-XXXX-XXXXXXXX" Version="1.0" />
And make it look like this:
<Identity Name="XXXXX-XXXX-XXXX-XXXX-XXXXXXXX" Version="126.96.36.199" Publisher="CN=XY" />
(Where XY is your name as publisher, for example knifegrindergames)
If you want to test on a real device remember to click on the arrow next to the play button and select "Windows Phone 8.1" as startup project (Windows 8.1 is the default option).
Said that, my first experience with WP 8.1 was quite disappointing with a lot of inconsistencies, audio drops and crashes, but since i make an heavy use of event sheet includes (r166 have a bug that reverts the usual order of fired events), VS 2013 is still a RC (not even RTM), WP8.1 is still a beta and I'm running it on a 512Mb device (a crappy Lumia 520)... well... I can't blame anyone or anything for that. I'll test soon on a proper device (a Lumia 920) building my project with C2 r167 and I'm confident that a lot of issues will go away.
Curiously Canvas2d mode seems a lot faster than WebGL.