repotting is best done in Spring, Ficus benjamina shouldn't normally have trouble standing up, so I can only assume that it has somehow been allowed to grow at an odd angle leading it to topple over like this. If you do have to repot it in order for it to stand upright then carefully remove the rootball with as little damage as possible and then put it upright in the same pot and pack the soil around to support it.

Timing is a primary consideration, often determining whether repotting or potting up is the wise choice. For you at this juncture, potting up if required is the wise choice. Repotting non-temperate plants, which includes removing all or most of the old soil and root-pruning, is best undertaken in the month prior to the plant's most robust growth period for a very high % of plants commonly grown indoors, yours included. That translates to July for you. While potting up can be undertaken at any time, you should probably avoid spring repots of Ficus, as the plant will most likely have the lowest energy reserve level of the entire growth cycle, ensuring a long recovery period and increased likelihood of susceptibility to insects and disease due to the additional weakening subsequent to repotting.

Repotting is an essential part of maintaining all trees in containers, if you expect the plant to grow at or near its genetic potential. Repotting ensures the plant will at least have the opportunity to grow to its potential within the limits of all other cultural influences, while simply potting up ensures it will not.

It is important to mechanically secure the plant so it will not move in relation to its container. A secure plant establishes/reestablishes in a fraction of the time that it takes for unsecured plants. If it was my plant, I would make some deep vertical cuts in the root mass with a very sharp utility knife, and score a deep 'X' on the bottom, or saw off the bottom 1/4 of the roots and pot up into a slightly larger container until next summer. Move the plant outdoors as soon as temps allow, acclimating it to full sun, & let the plant gain strength so it recovers from next summer's repot quickly.

You might find this info about tending Ficus trees in containers helpful.

And this thread that contains lots of info about trees in containers and repotting should have some info especially helpful to you.

Good luck. ..... additional questions?

Al

carlisleliverpool - Thank you for responding. Yes the previous owner did let it grow out at an funny angle, they didn't rotate it so that all sides had the same amount of light. We've pushed the tree back and adjusted the earth around it and packed in more soil. I've pruned it on the bushier side. It's standing up on its own now but still at an angle. I hope by facing the less bushy side to the window it will eventually correct itself.

Al, thank you for your advice. I shall wait till July for re-potting. Thanks for the links too!