in cPanel mailbox quota for all email accounts under a single domain name is stored in a file named quota.
To bulk change mailbox quota for all email accounts under one domain you can modify the quota file located in the following path:
/home/user/etc/domain/quota*Where user is your cPanel username and domain is the domain name (replace it with your-domain.com).
From this file you can edit the quota shown in bytes for each account individually or for all of them at once using a text editor with search&replace functionality (such as Notepad++) or using a script like this one:
cp quota quota.old
awk -F: -v OFS=: '{$2 = "1073741824"; print }' quota > quota.1024
mv quota.1024 quota