First of all, make sure you really want to lose the NUMBER column. Why are you changing the column? For example, if the column is temperature, and you're changing it so that you can enter values like '68 F' or '20 C', then maybe you should leave the temperature column as a NUMBER, and add a new column, temperature_scale VARCHAR2 (1). (Better yet, convert all the temperatures to one system on input, perhaps using a trigger or a virtual column, and save the original measurment, like '68 F' in a separate column, if you really need to know how it was entered.)
Given that you really do need to change the column to a VARCHAR2, then one way is
1) add a new VARCHAR2 column
2) populate this column from the existing NUMBER column
3) add a NOT NULL constraint to the new column (if needed)
4) drop the original NUMBER column
5) change the name of the new column to what the original column was (if wanted)
If you drop the whole table, then you lose all its indexes, triggers and grants. If you just add and drop columns, you don't lose those things.