If you want to update the existing leads of your brand from Google Sheets, the best way is to import them to your brand. To do so:
1. Download the Sheet as CSV. MailBluster only supports CSV format.
2. Upload it from Leads > Add new > Import leads page.
3. Make sure to enable Update existing leads, otherwise MailBluster will skip importing existing leads.
4. Finally, Import.
Please note: These are 3rd party apps and may charge you for using it.
The last approach requires programming knowledge. Here you need to read the Google Sheets file programmatically and call using MailBluster developer API to update the lead.