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:

https://lh5.googleusercontent.com/0Od3W3sy7nxQDY06EN22SjqdMuwzJBnF4I5zVPTSLkOZ2ycjLTri048ttVnzFTmya6IHKzlm3cYH9Zv-zN5cWmcwdK2e4Uq1OVfUPof24-5imaRju1uz1skGWDjSbBy4yrgY1Aoo

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

https://lh6.googleusercontent.com/wEW3bZ8F_MowOEJ1qyZT7vjBbb4H6pzU9Xnr9kEzJKmp8TXxqOox-ONmebCQSg4Ebvpogc0lscG9yi11yY_lHbcy_Xz0yVpA_mkimzAEUYI15NWSsM8OA9KqHxTZ2gVhVV-gDndz

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: https://docs.matic.network/docs/develop/metamask/config-polygon-on-metamask/#configure-polygon-on-metamask