Update custom module entry's field values
Here is our approach to update custom module entry's field values

Our Assumption
1. We assume that you have a custom module
2. Also you want to update a custom module entry when an entry is created or edited
Approach
We've given the steps to update custom module entry's field values
1. Create workflow to update custom module entry
1.1 Tap on Settings icon

1.2 Tap on "Workflow Rules" under AUTOMATION

1.3 Tap on "Create Rule"

1.4 Fill all mandatory fields as mentioned in the image and tap on Next

1.5 Choose "On a record action"

1.6 Choose the following as mentioned in the image and tap on Next

1.7 Choose 'All' and tap on Next

1.8 Choose "Function" for Instant Actions

1.9 Tap on "New Function"

1.10 Choose "Write your own"

1.11 Give Function Name as mentioned in the image

1.12 Tap on "Edit Arguments"

1.13 Choose Param value as mentioned in the image and tap on Save

1.14 Copy and paste the below Code Snippet and tap on Save

1.16 Tap on "Save"

Code Snippet
Related Community Question
Updating multiple fields with a workflow function | Link
