In Appian 7.3, when you have a list of one CDT nested inside another CDT, is it

As an example, let's say the CDT structure follows something like this:
{label:1, value:3, multiples:{{field1:2, field2:4, field3:6},{field1:1, field2:3, field3:5},{field1:8, field2:9, field3:10}}, info:8}

And this is all stored in pv!CDT. If you want to access one of the nested multiple CDT's fields, then you would use this:
pv!CDT.multiples[2].field2

This will return 3, the value of field2 at the second nested multiple CDT. From there, you can update the value as you want.

As an example, let's say the CDT structure follows something like this:
{label:1, value:3, multiples:{{field1:2, field2:4, field3:6},{field1:1, field2:3, field3:5},{field1:8, field2:9, field3:10}}, info:8}

And this is all stored in pv!CDT. If you want to access one of the nested multiple CDT's fields, then you would use this:
pv!CDT.multiples[2].field2

This will return 3, the value of field2 at the second nested multiple CDT. From there, you can update the value as you want.