Computer languages have to be very specific, so dialects of SQL sometimes have specific key words for this. I think Oracle uses MERGE, MySQL uses REPLACE, etc. (I haven't used either in a while so I may have those mixed around.)
I can't think of a word for this in general English because outside of information processing and record keeping, the two ideas are too far apart. You wouldn't normally say, "I will replace the gutters on my house, or if I don't presently have a house I'll build one." Or, "You should change the oil in your car every 7,000 miles, or if you don't have a car you should buy one." In general, updating a thing is a very different idea from acquiring a new thing.
But in record keeping, whether on a computer or on paper, it makes a lot of sense to say, for example, "Add 2 to the inventory count for this item. If there is no inventory record, create a record with a count of 2." I'm hard pressed to think of examples outside record-keeping. Maybe some one on here can think of an example.
I think the conventional thing to say in data processing, outside of contexts where the language has a specific keyword, is "create/update".