When you complete a mission on the campaign use your three skill 
points, but don't click on the big tick, click on the corner under
the bottom right hand side and there will be the corner of the 
tick which was the tick that bought you to the upgrade screen, 
this refreshes the upgrade skill page, keeping the skills you 
bought last time but giving you 3 new points (i only found this 
out when my mouse had a spasm when i clicked and clicked it, by 
the time i had finished the campaign i was level 500!