Is it possible to change the database user password directly in the web.config file?
The recommended way to change the database user password is to reconnect to the same database using the Administration and Monitoring Station > Settings > Application Server page. Please refer to the following Online Help topic for the details: Connecting to an existing database.
The web.config file stores the password in a form of a hash. The hash of the password is received in a way described in the following Knowledge Base article: How passwords are encrypted in FlexiCapture 12?
Once the password is encrypted, you may change it in the web.config directly, but this is not a recommended way to change the database user password.