Use https://www.stellar.org/laboratory/#txbuilder?network=public to build a transaction of the kind "Account Merge". Specify the account that will be merged as the source, the account that will receive the merge as the destination. Then sign the transaction by just pasting the secret key of the account that is being merged, and finally submit the transaction. The interface is quite self-explanatory.
You can try issuing some transactions with testnet accounts first if you're unsure.