Equipping weapons to use inside of Peakvox Ninja is a very easy thing to do. First, approach one of the three game portals. I suggest using the training lobby to change weapons, because this way you aren't holding up other players who want to battle in the beginner and normal arenas.
Press the X button to enter the game lobby. In the top left of the lobby menu you should see the "Members" tab, as well as several other tabs you can switch to using the direction pad on your controller.
Switch to the "Weapons" tab, which will bring you to a list of all the various weapons you own and can purchase for the game.
On the top right of the screen you will see how each weapon is assigned to a certain button on the d-pad.
Using the up/down buttons on the controller's d-pad, scroll down the list of weapons until you find the umbrella you purchased.
Once you've found it, press the X button and you will then be given the option to assign it to one of the four d-pad slots. Close range weapons can only be assigned to the bottom slot.
Press X on the slot you want to assign the weapon to (in this case it would be the bottom slot).
And there you go, your umbrella should be equipped and accessible in the game. Just tap down on the directional button to bring it out while you're playing. Hope this helps.
Weapons may only be equipped while in the matchmaking menu before a game begins.
The easiest way to do this is to access the Training Lobby on the right side of the Peakvox Ninja space. Once it has loaded, press right on the Dpad to scroll to the screen that shows your equipped weapons. Press down to scroll through available list of weapons. Press X to swap them.
Since you're new to the game, this guide may help you: