What exact version are you on? 7.0? 7.6?

If you are on before 7.5 , you need to migrate your assets with migration tool

No shortcut, the only way is the hardcore way, follow the guidelines here:

https://world.episerver.com/documentation/upgrading/Episerver-CMS/75/

Depending on you complexity and size of your site, you may need to upgrade to lower version, eg version 8 first, and then go to latest. 

Make a POC, You upgrade the nuget packages for solution, fix the compile errors, fix the runtime errors, test the site.

If you have commerce, it much more complex but in theory the same. 

Good luck!

Just gone through an upgrade from 7.0 - 10.5, it is a very manual process however once you are on 7.6 nuget makes everything a lot simpler.  As Gosso mentioned each step in each version of the upgrade documentation is pretty much essential.

Top tips:

After a version jump, clear out old dlls and Addons, they will be problem 99% of the time.

Episerver.Framework.config -> updateDataBaseSchema is a lifesaver

I had a problem with the sitespecific root block folder not being setup, this is solved by Admin -> Config -> Manage Websites, Untick, Save, Tick and Save, the Use site-specific assets checkbox for each site.

Update your MVC library, this gave me decent speed bump

The VPP migration tool did not like the web.config's <episerver.shell><protectedModules> so I commented them out.