Use a plastic leaf guard that will tuck under the first row of shingles, or add more gutterhangers/ screws use also a gutter sealant or caulk. This will fix an aluminium gutter leak problem. If copper gutters it will be best to call a professional
How To Stop Gutter From Leaking Between Gutter And Roof?
Buy a plastic leaf guard that tucks under the first row of shingles or add more gutter hangers/ screws used in combination with a gutter sealant or caulk behind the gutter (if aluminium). Copper gutters... Call a specialist.