Xcode 7.0 Beta Throwing Heaps of Errors, update OS X?

The first step is probably to go to the "Product" menu in your project in Xcode and hold down the option key and choose "Clean Build Folder...", to make sure there isn't any previously compiled code causing problems because of incompatibility.

I don't think upgrading OS X to the El Capitan would improve anything for you, since a lot of us are just running Xcode 7 beta on Yosemite.

If you haven't already backed up your project, you might want to do that before updating the code further, just in case you end up deciding to go back and wait.

Assuming there isn't isn't anything really weird going on with your project, you would just need to work your way through it fixing the errors and warnings generated by the compiler. There can be a lot of new errors and warnings because of the changes between Swift 1.2 and 2 and the updates to Cocoa, but most of the errors and warnings should be relatively trivial to fix (changing var to let, updating the old generic sort/map/filter/find functions to be called as methods, updating use of OptionSets to the new syntax, etc).

If your code relies on 3rd party (non-Apple) Swift code or frameworks, you'll end up needing to convert/update that code to Swift2 as well, or wait until updated versions are available.

FYI, It took me a full day to convert 10,000 lines of Swift 1.2 to Swift 2, after the auto conversion ran. As with any recent Beta, I have experienced a number of problems, the most annoying being the inability to reliabily run in simulator. It seems that I have to run from xcode, then it fails to connect. So I then go to the simulator and run the program manually, kill it and run it again manually (usually it kills itself the first time), then once I have it running manually, kill it, then run it from Xcode. And yes, I also get lots of stuff in the log that seems less than desirable. Lets hope that there are a bunch of stored up fixes just waiting for everyone to get back from WWDC.