Log in to your Phantom wallet
Click on Receive

Select Polygon

Copy the Polygon address from your Phantom wallet
Go to Step 2
Log into your Sinbad wallet
Ensure you have a balance. (Your balance is already in USDC)
Click on Cash out

Select USD Coin (USDC) option

Enter the amount you want to withdraw then click on continue

Paste the Polygon address you copied in Step 1 above