You don't have to leave your minimum balance stranded in an old account. Build a merge transaction in the Stellar Laboratory.
Note: Do not merge your account with an exchange address - you will likely end up losing your lumens. Remove all trustlines on the old account before merging.
Visit the Transaction Builder in the Stellar Laboratory - tools for interacting with the Stellar network:
Make sure you have selected “Public” at the top right of the page.
In the 'Source Account' text field, paste the public key of the Stellar account you want to delete (and merge).
After you paste the public key, a blue button will appear that says “Fetch next sequence number for account starting with…”. Click it to populate the text field with your next transaction number.
Skip down to the 'Operation Type' section and select 'Account Merge'.
Paste in your desired main Stellar Account ID in the 'Destination' text field, leaving 'Source Account' blank.
Click the button at the bottom left of the page that says “Sign in Transaction Signer”.
On the next page, scroll to the bottom and either paste in your private key in the text area next to “Add Signer” or sign in with your Ledger Wallet.
If you are using your Ledger Wallet, you will need to confirm the transaction on your hardware device.
After signing the transaction, scroll down and click the 'Submit to Post Transaction endpoint'. This will submit your merge request to the Stellar network.
Check your old and main accounts to make sure the operation was completed!