Assuming you have connected to Google Merchant Centre using Shopify's Google App/Sales Channel, you can disconnect it as follows:
Go under - Sales channels > Google > Settings > Google Merchant Centre account
then, if connected via the Google sales channel, you should see the option to disconnect from Google Merchant Centre as presented below:
** Please note, that disconnecting under Google Merchant Centre alone will cause Shopify to relink itself unless disconnected under Shopify as explained above.
In case you can't see the option to disconnect, that means you might have connected with Merchant Centre using a 3rd party app instead of Shopify's Google Sales channel as presented above in which case, you will have to disconnect as explained by the relevant app's developer.