Change values from one column higher than X but not change the others

How can I change all values from a column higher than 30 in 30, but if there are lower keep the value that it has?

Day Temperature
1            25
2            32
3            28
4            35

I want to have

Day Temperature
1            25
2            30
3            28
4            30

I have tried with ifelse but it also changes the other values.

Thanks in advance