how to override shortcut for multiple cursors sublime text 3?

Question

I am using linux [FEDORA 20]. I want to override shortcut key for multiple cursor which is
ctrl+alt+up/down
because it is used for switching workspace in f

Answer 1

CtrlAlt↑/↓ is the keyboard shortcut for multiple selection in Windows, not Linux. The following is from the Linux keymap file:

{ "keys": ["alt+shift+up"], "command": "select_lines", "args": {"forward": false} },
{ "keys": ["alt+shift+down"], "command": "select_lines", "args": {"forward": true} },

Answer 2

Go to Preferences > Key Binding - User
and add following lines to override default key bindings.

{ "keys": ["ctrl+space+down"], "command": "select_lines", "args": {"forward": true} },
{ "keys": ["ctrl+space+up"], "command": "select_lines", "args": {"forward": false} }

where

ctrl+space+up/down 

is your key bindings.