Having a 1% increase means we go from 100% to 100%+1% = 101% which converts to the decimal form 1.01; note how you move the decimal point two spots to the left to go from 101% to 1.01
A 1% decrease means we go from 100% to 100%-1% = 99% = 0.99
Multiplying 1.01 and 0.99 leads to 1.01*0.99 = 0.9999
Since 0.9999 = 99.99%, this means we have a 100% - 99.99% = 0.01% decrease overall.
Note: the order of the decrease and increase doesn't matter. You could do the decrease first, then the increase, or vice versa.
