You automatically earn climate credits (O1/O2) as your trees grow so don't need to do anything.
If you want to transfer the credits or keep them in your own wallet, you can follow these steps to claim them from smart contracts:
Visit your forest on Treejer
Connect your wallet
Click on "Send to Wallet" button next to your climate credits
Confirm the transaction in your wallet (For Metamask users, a new window pops up)
Done! Please wait a few moments until your transaction is confirmed in the background
You climate credits must have transferred to your wallet after the transaction is confirmed. If you can check your O1/O2 balance in the profile box on your forest page. It's located in the right sidebar.