Code:
SQL> startup
ORACLE instance started.

Total System Global Area  105978600 bytes
Fixed Size                   453352 bytes
Variable Size              83886080 bytes
Database Buffers           20971520 bytes
Redo Buffers                 667648 bytes
Database mounted.
ORA-01157: cannot identify/lock data file 2 - see DBWR trace file
ORA-01110: data file 2: 'D:\ORACLE\ORADATA\TEST\UNDOTBS01.DBF'

SQL> alter database datafile 'D:\ORACLE\ORADATA\TEST\UNDOTBS01.DBF' offline;

Database altered.

SQL> alter database open;

Database altered.

SQL> alter tablespace UNDOTBS1 
add datafile 'D:\ORACLE\ORADATA\TEST\UNDOTBS02.DBF' size 200M;

Tablespace altered.

SQL> alter database datafile 'D:\ORACLE\ORADATA\TEST\UNDOTBS01.DBF' 
offline drop;

Database altered.
SQL> shutdown immediate;
Database closed.
Database dismounted.
ORACLE instance shut down.
SQL> startup
ORACLE instance started.

Total System Global Area  105978600 bytes
Fixed Size                   453352 bytes
Variable Size              83886080 bytes
Database Buffers           20971520 bytes
Redo Buffers                 667648 bytes
Database mounted.
Database opened.