You can visit any etisalat Business Centre.
If you add a data package to your primary service, the data will be shared between the SIMs as part of the Multi SIM service.
If you migrate from a postpaid to prepaid service or vice-versa, the Multi SIM service will be deactivated when you request for migration of service.
If you want to upgrade/downgrade the data package or the primary service, it will not affect the Multi SIM service.
For example, if you want to migrate from the 1GB plan to a 5GB plan, then the data + voice as per the new plan that you migrate to is shared between all the SIMs you are subscribed to.