At its meeting, the board of equalization shall proceed to equalize and correct the assessment roll. It may change the valuation and assessment of any real property upon the roll by increasing or diminishing the true and full valuation thereof as is reasonable and just to render taxation uniform. All complaints and grievances of residents of the city must be heard and decided by the board and it may make corrections as appear to be just. Complaints by nonresidents with reference to the assessment of any real property and complaints by others with reference to any assessment made after the meeting of the city board of equalization must be heard and determined by the county board of equalization.