The files attached to messages should be more or less static files such as pictures, lists of assets, wills, policies, bank account information (...). As such, like you would proceed with attachments to regular email messages, If you need to change a file you have already attached, you would have to detach the previous version of the file and attach the new version.
As a solution, If you have frequently updated work files that you want sent to someone in their latest version, the best option is to save the files to a folder in an online storage account (e.g. DropBox, OneDrive, iCloud, Mega...) and share a link to that folder in a message. This way, the recipient of the message will always get the updated files.
The specific details of this operation will depend on the type of storage service you use, so you should be familiar with them.