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Change the MX record to the ones provided by Google
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Change your NS records to Google IP address
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Create a new A record
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Check and lower the TTL
EXPLANATION
It's always a very good idea to check and lower the
Time-To-Live (TTL) for a domain to ensure that any changes made are
propagated in a short time frame and your customers and boss remain
happy. NOTE: TTL is normally specified in seconds, you need to divide by
60 to get minutes.
https://mediatemple.net/community/products/dv/204644120/understanding-ttl-(time-to-live)
In the end you will need to change the MX record, but changing the MX record does not change how quickly the change will happen.