How to have left hand mode

put cl_righthand 0 into the console you can access the console with the ~ buttom above tab

if you would like it back replace the 0 with a 1

If you would like binds

bind key "cl_righthand 0" //This is for Left hand
bind key "cl_righthand 1" //This is for Right Hand

For me by binds are
bind h "cl_righthand 0" //This is for Left hand
bind j "cl_righthand 1" //This is for Right Hand