[R] regroup row names

[lily li]

I have a problem in changing row names in a dataframe in R. The first
column is ID, such as aClim_st02, aClim_st03, aClim_st 05, bClim_st01,
bClim_st02, etc. How to rename the names, so that aClim_ all grouped to
aClim, while bClim_ all grouped to bClim? Thanks for your help.
df

ID                    temp   precip   LW   SW
aClim_st02
aClim_st03
aClim_st05
bClim_st01
bClim_st02
...

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