Click on the 3 line icon in the top right corner and select Compliance, then choose the Data Cleansing option on the right. On this page you can switch between People, Company and Project duplicates. For each of these, there is a Manual Matching button on the right. Click this to get a new pop-up
Enter the name of the record to be merged in both the left and right fields, and pick the duplicates from the lists – the one on the left is the master record whose values will be retained if there is a conflict.
If you don't have access to the data cleansing screen, you will need to ask an administrator to merge the records.