Find exact formula of a curve and fix the bumps in the curve

sampledata.xlsxI created a chart from my data points, and my excel "curve" is flat at the beginning and bumpy at the end. I want to be able to adjust the curve so that the beginning is not flat and the end is not bumpy. I am new to statistics, and looking for a way to adjust the data. Any help would be appreciated. Thanks. Also would like to get the exact formula of my bumpy curve not of the trendline. Thanks.

I attached a sample file (not my exact data, but a similar situation. So how do I find the exact formula of this curve and how to fix it to eliminate the flat and bumpy. Thanks.