Correct spelling, explanation: investor is the correct form of this noun. This is due to the fact, that originally the word comes from Latin (investire) and as such it receives the suffix -or. There is also another noun suffix, which relates to a person, -er, but generally, we use -or for words deriving from Latin. Therefore investor is the correct spelling, not invester.

Definition of investor:
noun, someone who invests
The investor resigned so we were in a stalemate.
Michael wanted to be a stock investor but he wasn’t really good and resigned quickly.

Incorrect spelling, explanation: invester is a mistaken form because a wrong suffix was used. In English, there are some noun suffixes relating to a person (-er, -or, -ist, -ian), but it is widely accepted that -or is used for Latin-originated nouns. Therefore investor is the correct form here (Latin root: investire). Investor is the correct spelling here.