This is done from Manage Earmarked Reserves under the Setup menu (only available to Clerk/RFO and Administrator).
You are presented with the following screen
The existing reserves are displayed with options to rename or edit. There is also a button provided to create a new reserve. When this is pressed, the following screen appears
Just enter the name of the reserve in the box as required. Note that the name must be unique and so, if you enter the name of a reserve that already exists, an error will be flagged up.