How can I remove dark spot on forehead?

Depends. If it is simply sun-damage, some topical treatments or some laser treatments may a good option. However, if the lesion is concerning for melanoma, it needs to be biopsied. Melanomas are screened by the abcd's: a-asymmetry; b-irregular borders; c-different colors; d-diameber >6mm (size of pencil eraser). If you notice a mole with these characteristics, get it checked out asap.
IPL. Usually ipl (intense pulsed light), otherwise known as photofacial procedure works well for dark spots. Hydroquinone is a topical bleaching cream that works well also.
Mole? Hyper pigment? It is impossible to tell if this is just color change from inflammation which can be lightened with peeling cream such as retin a (tretinoin) or a growth that would need to be removed.Since a dark spot may be abnormal have it evaluated.
Biopsy. Rather than thinking about removing it, make sure it is benign first. This can be done by seeing a plastic surgeon or dermatologist. A biopsy may be in order.
Few options. Skin lightner such as hydroquinone with ipl laser is the best combination.
Dark spot face. Best to see in person a derm or ps. You may need a biopsy so be aware of this possibility. If only pigmentation either a peel solutions or laser therapy could help remove.