We've had a lot of rain and little sunshine for the last 3 weeks.
Geranium Leaves Are Turning Yellow
Certified GKH Gardening Expert
The excessive water and less then the ideal 6 to 8 hours of sun most likely is the issue.
You can remove the yellowing leaves.
Make sure your pots or containers are well draining so the plant is not sitting in soggy soil.
Here is a link.