systemd is used instead of init and hence the concept of runlevels is replaced by the term targets. So there is indeed a mapping between init-based runlevels and systemd-based targets:

Mapping between runlevels and systemd targets

                                                                            
                                            ┌─────────┬───────────────────┐ 
                                            │Runlevel │ Target            │ 
                                            ├─────────┼───────────────────┤ 
                                            │0        │ poweroff.target   │ 
                                            ├─────────┼───────────────────┤ 
                                            │1        │ rescue.target     │ 
                                            ├─────────┼───────────────────┤ 
                                            │2, 3, 4  │ multi-user.target │ 
                                            ├─────────┼───────────────────┤ 
                                            │5        │ graphical.target  │ 
                                            ├─────────┼───────────────────┤ 
                                            │6        │ reboot.target     │ 
                                            └─────────┴───────────────────┘ 

Now, to just change the „runlevels“ in 16.04, you can use for eg:

sudo systemctl isolate multi-user.target 

To make this the default „runlevel“, you can use:

sudo systemctl enable multi-user.target 
sudo systemctl set-default multi-user.target 

From man systemctl

   isolate NAME  Start the unit specified on the command line and its dependencies and stop all others.  
                 If a unit name with no extension is given, an extension of ".target" will be assumed.     
                 This is similar to changing the runlevel in a traditional init system. The isolate command
                 will immediately stop processes that are not enabled in the new unit, possibly including  
                 the graphical environment or terminal you are currently using. 

Also have a look at man systemd.special to know more about the targets in systemd.