Hi Lyne,
If you want to change database fields, then best way is to go in Set Datasource Location and map old fields with new datasource fields. This will change all your report fields wherever it was placed.
Also you can rename formula names and that will reflect in all places.
Otherwise :
You need to right click on each field and say find in formulas then go in each formula and replace with new one.
Thanks,
Sastry