What year is your boat?
Catalina 22's from about 1985 on have internal halyards while earlier boats have external halyards unless a PO changed them to internal.Nevertheless, it seems from your question you already have internal halyards.Assuming you have wire with two eyes with line attached, you'll need to remove the sheaves from the truck (the thing on the top of the mast), tie messenger lines to the wire eyes at the truck, remove the turning blocks at the bottom of the mast and pull everything out. Turn the wire over end for end being respectful of which is for the jib and which is for the main, attach new line to the now "new" bottom eye of the wire and pull everything back up via the messenger lines. Reattach the turning blocks on the bottom of the mast and put the sheaves back into the truck. Viola! Done.