As Pat has pointed out

The meaning of “up to now” can depend upon the circumstances

Point 1: If it’s adjectival phrase it has a temporal significance (and can be freely varied by use of “up until / till now”) – in which case it is a classic adjective signal for the use of the perfect aspect (as in present perfect tense)

Point 2: If it’s a use of the phrasal verb “to be up to something” then no variation “to/till” is possible and the meaning is similar to doing.

At first glance, I assumed you were referring to the adjectival phrase (point 1) but as Pat has identified, in the absence of further context, one cannot be sure which is the correct answer to your question