What is the difference between regression beta and bottom up beta?

The regression beta uses the historical stock price for the company. Usually 5-year monthly data, and you run a regression to obtain beta. On the other hand, bottom-up beta uses publicly comparable firms then you obtain  median or average leveraged beta. Also, estimate the average market value (D/E ratio). After that leverage it. use the following formula: 

Beta unleveraged = Beta levered/ (1+(1-tax rate)*(D/E of comparable))

Beta firm = Beta unleveraged*(1+(1-tax rate)*(d/e of firm))