Edit the name of existing sprint in Azure DevOps?

First Method

If you are using the new navigation go to your

Select "Project settings" in the Lower left corner of the project View.

Project-settings -> Boards -> Project configuration -> 'Iterations'

Now you should see your iterations. Right-click the iteration To Edit.

Second Method

Boards -> Sprints -> Click Date-range

above the Sprint name near the upper right side of the screen to edit the name of the current sprit

You can do that in following way:

Click on Project-settings ==> Boards ==> Project configuration ==> Iterations

See the screen shot for better clarity.

Alternative Way:

You can also do it by simply following steps:

Boards ==> Sprints ==> Set Dates (Fill out date range here)

See the screen shot for better clarity:

Hope that would help