The Update a Contact Property action allows you to update the value of certain types of custom fields on those contacts who enter the Journey's funnel. See Fig 1 below. 

Fig 1

To select which custom field to update, click the Update a Contact Property action node once you have added it to the Journey work area. Doing so will bring up this popup ("Properties") on the right-hand side of the screen (See Fig 2 below):  

Fig 2

Clicking the "Custom Field" input field below the "Update a Contact Property" label will bring up a list of all your available custom fields that are subject to the Update a Contact Property action (See Fig 3 below). Select the custom field by clicking its name.

Fig 3

Clicking into the "Operation" field will allow you to perform various math operations on the selected custom field, the specific operations allowed dependent on the custom field type:

• "Date" field
  • "Reset to Null" - will set the value to "NULL", which will result in an empty field record
  • "Delete Value" - will remove the value in the field, which will result in an empty field record 
  • "Change to:" - will change the field's imported value to this value
  • "Changed fixed to:" - - will change the field's imported value to this value
• "Numbers Only" field - you may add, subtract, multiple or divide the custom field value by an value, which can be either an integer or decimal value.
  • "Reset to Null" - will set the value to "NULL", which will result in an empty field record 
  • "Delete Value" - will remove the value in the field, which will result in an empty field record
  • "Change to:" - will change the field's imported value to this value

See Figs 4 & 5 below.

Fig 4

Fig 5

Once you have finished with the action's setup, click the "Save Properties" button.