To round to the nearest tenths:
For example,
34.56
The number 5 is in the tenths place so you want to round by the number to the left of it which is 6. Now since 6 is greater than 5 the number will go up once. Giving you,
34.6
To round to the nearest hundredths:
For example,
57.839
The number 3 is in the hundredths place so you want to round it by the number left to it which is 9. Since 9 is greater than 5 the number will go up once. Giving you,
57.84
Note:
The number has to be greater than or equal to 5 to make it go up once but, if it's less than 5 the number will stay the same.