Do I need to create index for the column when using it in update statement?

I have a table in postgresql11 which has a userId as a column. And it is not primary key in the table. I need to perform below update statement on the table.

update entity set name='xxxx' where userId = 'yyy.

The userId is in the where condition but I don’t know whether I should create an index for userId. Will it improve performance? If yes, how can I specify index in the update statement?