How do I fix upper thread tension being too high?
Are you using the correct needle point and size for the fabric? 


Ball points for knits 
Straight points for woven fabric


Sometimes the needle just needs to be changed.

The other problem could be the thread holder, are you using a spool or cone? How is it attached or docked to your machine or the table?  
 
If it is a spool that is loaded sideways, the thread might be getting caught in the spool's edges and you just need the slide on piece (spool cap) that holds the spool in place. This may also help on a vertical holder.   

If you are using a cone, check to see if the thread is not releasing, sometimes it can get caught up underneath. You may want to get a spool holder and mount it to your table near the back of the machine.