change all values in a dataframe with other values from another dataframe

You can use DataFrame.replace() to do this:

df2.replace(df1.set_index('val')['num'])

Explanation:

1. The first step is to set the val column of the first DataFrame as the index. This will change how the matching is performed in the third step.

2. Convert the first DataFrame to a Series, by sub-setting to the index and the num column. It looks like this:

   val
   1    0
   2    1
   3    2
   4    3
   5    4
   Name: num, dtype: int64
   

3. Next, use DataFrame.replace() to do the replacement in the second DataFrame. It looks up each value from the second DataFrame, finds a matching index in the Series, and replaces it with the value from the Series.

Full reproducible example:

import pandas as pd
import io

s = """   val  num
0    1    0
1    2    1
2    3    2
3    4    3
4    5    4"""
df1 = pd.read_csv(io.StringIO(s), delim_whitespace=True)
s = """   0  1  2  3
0  1  2  3  4
1  5  3  2  2
2  2  5  3  2"""
df2 = pd.read_csv(io.StringIO(s), delim_whitespace=True)

print(df2.replace(df1.set_index('val')['num']))