You can set using boot statement. Here is an example
3500#dir
Directory of flash:/
2 -rwx 1810753 Mar 01 1993 19:07:14 c3500xl-c3h2s-mz.120-5.WC13.bin
3 -rwx 796 Mar 01 1993 01:56:37 vlan.dat
5 -rwx 1280 Mar 01 1993 02:00:25 config.text
6 -rwx 43 May 03 1993 01:39:27 env_vars
3612672 bytes total (1797632 bytes free)
3500#conf t
Enter configuration commands, one per line. End with CNTL/Z.
3500(config)#boot system flash:c3500xl-c3h2s-mz.120-5.WC13.bin
3500(config)#end
3500#wr mee
2w6d: %SYS-5-CONFIG_I: Configured from console by consolm
Building configuration...
[OK]
3500#