I have got 29er tires and I want to upgrade to 29 plus
Factors that decide whether or not a certain tire width fill be compatible with your bicycle are many. This applies to any transition that keeps the tire diameter (29 → 29+, 27.5 → 27.5+, 26 → 26+). The most important, in my opinion, are the following.


Your frame and fork clearances should allow wider tires. Both in increased width and height. Typically, bicycle vendors designate if a bike is "plus" compatible. If not, the risk is high that no tires designated as 29+ will fit in a 29 frame. In such case, you'd better consider switching to 27.5+ wheels if you are firm to go plus.
Your rim width. There are limits to how wide/narrow a rim can be to accept a given tire width. Here's ERTRO guidelines, copied from here:




Having a too wide or too narrow rim would result in a suboptimal performance or even straight unstable wheel. Another picture from 1:




Actual tire dimensions when installed on a given rim may differ from what's designated on it. There are a lot of reports online that a 2.6" tire actually measures closer to 2.7" or even 2.8". The same applies to non-plus tires: a 2.1" can sometimes be 2.25" when installed on a wider rim. Knobs can add to width and create problems with clearance. I recommend either playing it safe and taking a narrower "plus" or finding an online review with actual measurements of chosen tire applicable to your situation.


Other factors are less deciding. Inner tubes should be fine. There might be some extra muscle effort needed if you install tubeless-ready tires on a regular rim (or was it the other way around? cannot remember).

If your fork does not accept wider tires, you can change the fork. If it is the frame that has too narrow rear wheel clearance, changing it equals practically to obtaining a new bike. Getting new wider wheels is easier, because you'll then have two pairs of nice wheels. Some people use 27.5+ wheels on 29-ers, but if bike geometry is not an empty sound for you, you might consider doing some calculations before going that way.