This tutorial will show you how to send XRP with the XUMM wallet.
Step 1:
Unlock your wallet, press the blue xApp button on the bottom of the screen, and scan the QR code of the wallet you wish to send XRP to.
Step 2:
Select the asset you wish to send, in this case, XRP, and enter the amount of that asset you wish to send. Then, press next and confirm the address of the destination wallet is correct.
Step 3:
Review the details presented and slide to submit the transaction.
It is always smart to try to send a small amount of xrp (0.5 xrp for example) and confirming that the transaction works properly before sending larger sums.
A sent XRP can not be retrieved by Anybody unless you ask the receiver to send it back to you (which the receiver obviously might not want to do).