For the sake of this article, I'm going to you already have logged in to the control panel and have access to modify other members. If you don't, and would like to have that access, please let us know!
I should also note that these instructions are for ExpressionEngine 3.
1. Click the 'Members' link located in the upper left of the main navigation.
2. Locate the member you could like to modify from the list, and click the edit icon located in the same row.
3. Click the 'Username & Password' link located in the sidebar on the left.
4. Enter the new password in both the "New Password" and "Confirm New Password" fields.
5. Enter YOUR password (not their previous password, but your actual password) in the "Enter your Password" field. ExpressionEngine asks this to make sure you actually have the privileges to reset their information.
6. Click 'Authenticate & Save'. A green alert should display confirming that the 'Member Profile Saved', and your'e done!
If you have gone through these instructions and are still having issues, please open up a HelpDesk ticket so we can help get this taken care of for you.