Yes,

Via Telnet or SSH send command below and box will update from the feeds to the newer version,

opkg update && opkg upgrade

Naturally we are talking about same version / improved version, NOT from 4.0 to 6.x++ version, these are different versions (Same is from same tree/descent, like 4.0 from 15 july, then 4.0 from 2 august etc) !

If you want to go from the 4.0 to the 6.x++, you need to reinstall/flash the new image NFI file.

TS