You are updating the clustered index, as the clustered index *is* the table. All of the columns in the table are part of the clustered index (not the key, but in the index)

There's nothing at all wrong with a 98% cost for the clustered index update. The costs are percentages, they have to add to 100, so all that tells you is that the only part of the plan that has a significant cost is the clustered index scan.

Don't look just at the %. Is the query slow, that's got to be the first thing to check. Also, the goal of tuning is not to change percentages of operators in a plan. It's to make the query faster.