There are a couple of ways to do this.
The easiest in most cases is to use the iPhone native app as follows.
Login as usual on the mobile app
Either scan the assets if they are nearby and there are not too many OR do a search to return all the assets you wish to update. Be careful to ensure you can get a list of results that are identified with a common data element that you can search for. For example, they might all be Type:Asset1 and you would search for Asset1.
Then at the bottom of the screen click the Update All button. This will take you to a blank asset detail screen where you can add the data fields you wish to update. Black fields will not be changed.
The system will confirm the number of assets updated
BE CAREFUL WITH THIS POWERFUL FEATURE AS YOU CAN QUICKLY ERASE/UPDATE A LARGE NUMBER ASSETS BY ACCIDENT TOO.
This feature works with the custom data fields too