BACKUP everything first Mark.
Your connection should be in:
/app/modules/connections/...
Just open the file in a text editor and make the changes required.
There are also database connections in the Targets too (remember the wappler directory is hidden so if you have not already done so enable show hidden files and folders):
/.wappler/targets/...
Each Target will have a database directory and in there are the database connection files. At the very end of the file below the structure is also a connection string.
I suppose the alternative would be to entirely delete the database connections from within Wappler but this may leave some residual files? I'm sure we had that issue in the past. Even now after years of using Wappler I have no idea why there is an Advanced tab, or why there are multiple database configuration dialogues within Wappler, ie in the main menu on the left, and under Server Connect Globals area...? So if you do make changes within Wappler (and not the manual changes I outline above) make sure to check both sets of configuration details match and are for the working (new) connection.
Also the Project Settings area (under Targets, and specific to each target) has a database configuration (so make sure to make changes there especially if you have selected Docker for the database type).
Once again just BACKUP first so if it all goes Pete Tong you can rollback without too much hassle.
It would be nice if in Wappler 8 there is one area to configure the database connection (per Target of course) rather than multiple database connection entry points as it is just confusing and makes no sense especially to a new User trying to figure it out for the first time...