How update an old phone app?

My older phone has Android version 4.4.4 and therefore can't get the discordapp update though google play store. It currently has discordapp version 3.9.0 (388)

I do have a newer Android tablet that can and does have the latest discordapp version, but sometimes I'm using it for other things so it's nice to use my older phone.

So, I'd like to update my older phone to the latest compatible discordapp version, how do that?

It does still work, but it's noticeable