Unfortunately, it is not possible to modify a pending or registered trademark. Once you file your application, you cannot officially add or change any element of a given mark. Simple administrative changes, such as the address of the trademark owner or a transfer of ownership, are possible; however, the trademark itself cannot be modified.
The reason for this is that the approval of an application hinges on a rigorous assessment done by the intellectual property office, where the examiner first evaluates the distinctiveness of the mark and then compares it against all previously registered trademarks to make sure no similarity conflicts exist. Modifying the mark would invalidate the result of this assessment and could be potentially used to bypass rules.
It is good to know that all 'confusingly similar' versions of your trademark are protected by your trademark as well without you applying for a second time. If a third party uses a slightly different version of your trademark, you can still ask them to stop, as it would be considered trademark infringement. Similarly, you can use a slightly different version or a variation of your trademark yourself; however, the changes should be minor.
In case of any major change, you should file a new application to fully protect the brand from being copied.