Word meaning both create and update?

"Save" seems perfect for this. As a developer I'd read "SaveRecord" as either inserting or updating the record depending on its preexistence.

In SQL terminology merge term is used for this: https://en.wikipedia.org/wiki/Merge_(SQL)

This is a synonym of upsert (update + insert) which, in my opinion, suits even better, because its meaning is explicit.

Since you stated you're dealing with JavaScript, I'll offer 'assign' as a handy verb. In most programming languages, you need to declare an object property before assigning it, but in JavaScript the assignment operator ('=') both updates and creates properties (when they don't exist).

So if you have your generic object here, and you want to create/update properties of that object using a single function call, you're essentially assigning values to the properties of the object. Creation is implied.

 myObject.prototype.assignProperties = (property, value) => this.property = value;

The only activity carried out in the function is 'assign' by the assignment operator. If the property doesn't exist, it is created.

Source: I'm a software engineer working with JavaScript regularly (FireFox OS applications).

Sometimes set is used in programming to refer to both creating and updating. It can depend on the language - some languages clearly separate the two operations, some do not.

Some languages try to be clear by not using a single verb for this. For example, SQL uses the verb CREATE OR REPLACE to mean exactly what it says (and what you said).

My suggestion is to check with your language and its users, to see what vocabulary is typically used for this in the particular context.