Assuming that you have already added the network and custom RPC for Matic mainnet in your Metamask wallet here is how you can switch:

  1. Open your Metamask wallet and click on the network dropdown to expand as shown in figure:

  1. Once the window expands you can select Matic Network to switch.

You have now switched to Matic mainnet.

You can refer to this link if you are looking for instructions on how to add the network to Metamask: