Arduino YUN Rev. 2 power supply
For version 1 VIN is connected, through a Schottky diode, to the 5V rail.  That means you can only provide, at absolute most, 5V plus a little bit for the diode drop into VIN.  The 5V pin can only source power on the Yun, since it is also connected to the 5V rail through a Schottky diode.

In effect the two pins form the "In" and "Out" of the 5V power.

However in version 2 the 5V pin's diode has been removed, and the VIN's diode has been reversed.  VIN is now known as "5V_OUT".

So the 5V pin can be used for both in and out, and what was VIN can only be used for power out.

To provide more than 5V to the Yun you will have to first pass the voltage through a regulator of some form.  Since you want to use 7.4V it sounds like you are using 2S Li-Ion (or similar) batteries.  Because of that I'd recommend a switching regulator, rather than a linear one.  I commonly use 5V UBEC modules from my favourite RC model supplier.

For version 1 you will provide it to VIN.  For version 2 you will provide it to 5V.  Quite why they'd change something quite so fundamental as the power pin arrangement from one version to another is beyond me.  It would cause havoc with compatibility between versions with certain shields.