Keep follow two steps:

1. Detect the the raindrop on image => use threshold algorithm (because the raindrop is lighter than others, so threshold algorithm is the simplest).

2. Use any inpainting algorthms to remove the above detected regions (example Mumford-Shah).

You can try anisotropic diffusion!

Dear Nor Atikah Abu Bakar,

You can use YCbCr color space for this work. YCbCr color space separate luminance from chromatic. Y is luminance component,  Cb and Cr are chromatic components.

try morphological tophat transform

Keep follow two steps:

1. Detect the the raindrop on image => use threshold algorithm (because the raindrop is lighter than others, so threshold algorithm is the simplest).

2. Use any inpainting algorthms to remove the above detected regions (example Mumford-Shah).