To migrate your ERC-20 MAIN tokens (Ethereum tokens) to BEP-20 MAIN tokens (BSC tokens) you need to follow 6 steps:
Install the MetaMask wallet and connect it to the address with your ERC-20 MAIN tokens. You may find the following articles helpful:
In the web version of the platform, go to your tokens page, click add funds button and select the Ethereum tab.
Deposit your balance with MAIN tokens, following the instructions on the page. Select your Ethereum address within MetaMask, connect it to the site, give permission to the site to use your MAIN tokens (the confirmation transaction will require network fee in ETH), enter the amount and sign the transaction within MetaMask.
After the transaction is confirmed and the balance on your account is replenished, go to the send tokens page (through your balance page) and select the send to BSC tab.
Next, following the instructions, enter BSC address to which you want to receive BEP-20 MAIN tokens, enter the transfer amount (note that you will additionally be charged a commission for the transfer in MAIN tokens, which covers the network fee), and confirm the transfer.
Wait for MAIN tokens to be sent to your BSC address. Please note that the transfer is not instantaneous and may take up to 5 business days.
The migration scheme from BEP-20 MAIN tokens (BSC tokens) to ERC-20 MAIN tokens (Ethereum tokens) is similar to the migration scheme above with the only difference that first you have to deposit your account with BSC tokens, and then transfer them to the Ethereum address.
Also, to fund your account on the platform with BEP-20 tokens, you will need to manually add the Binance Smart Chain to your MetaMask wallet. You can read how to do this here.