Here is one way to do it interactively:
1. Select 75% of rows by entering 0.75 in Rows -> Row Selection -> Select Randomly...
2. Make a subset of these 75% by selecting Tables -> Subset (check Selected rows and All columns in he Subset dialog)
3. Activate the original table (with the 75% still selected) and choose Rows -> Delete Rows to get rid of these rows and keep the remaining 25%.