First, create the new Matter where you will need to move the entries.
Before proceeding, ensure any invoices against the Matter being moved are voided, and any payments unallocated.
Click under Time>Search.
In the search criteria fields, leave the date fields open and specify the Matter you want to move to another Client, and hit Search.
Select the topmost checkbox to select all entries.
Click on Copy/Move button on the top right.
You can now specify the Client-Matter to move to.
Click on the 'Move' button.
Now all Time Entries have moved to the other Client-Matter.